Only difference is lighting and venue...
That's all the difference you need.
Addendum: Now that you've posted example images, we can point out some other differences as well.
In the earlier image on the left, there's far more light falling on your subjects than in the image on the right. At the same time, there's far less light in the background. The brightest things in the frame are the two contestants. All of the noise in the background is swallowed up by the dark background being reduced to pure black by your camera's processing engine (or your raw convertor's processing engine).
In the image on the right, there's far less light falling on your subjects, at least from the direction the camera is pointing. But there's more light in the background. The spotlight in the middle of the frame above the left glove of the boxer in red is causing the camera to meter the scene much brighter than it otherwise would. Instead of exposing for the subjects, as it did in the first example, it's exposing to not blow out the spotlight in the second. Your subjects are much dimmer in the second image than the first. Other than the spotlight, the background light is still pretty weak, but it's too strong to show up as pure black against your dimly lit subjects that aren't much brighter than the background. So all of the noise in those dimmer areas gets amplified and shows up in the darker, but not totally dark, parts of the image.
Notice the shadows of the boxers? Most of the light on them is coming from the other side of the ring. In other words, the sides of the boxers away from the camera are brighter than the sides of the boxers in view of the camera. If you'd shot from the other side of the ring, at least your subjects would have been brighter and less noisy compared to the background.
Noise is not a direct function of what ISO setting you used. Many people believe that it is, but it isn't.
Noise is a function of the ratio of usable signal, that is light, to the amount of random signal and signal not created by light entering the camera and being recorded by the camera's sensor.
In other words, the ultimate determiner of how noisy an image looks is the signal-to-noise ratio, a/k/a SNR or S/N ratio. Signal when talking about digital camera sensors is just another word for light. The more light you allow into the camera, the more signal you have. The less light you allow into the camera, the less signal you have.
How much light you allow into the camera is determined by three things:
- The entrance pupil of the lens a/k/a the "aperture"
- The length of the exposure a/k/a the "shutter speed"
- The intensity of light falling onto the front of the lens from within the camera's field of view a/k/a "scene brightness"
The more light there is landing on the front of the lens, the more signal will enter the camera at the same aperture (Av) and exposure time (Tv). When the scene is dimmer we must either open up the aperture or lengthen the exposure time to allow the same amount of light into the camera.
Notice that ISO doesn't appear anywhere in the determination of how much signal the sensor collects. ISO affects how much electrical amplification is applied to the signal after the sensor collects it. But this electrical amplification also applies to any noise already included in the sensor's output at the time the signal is amplified.
Noise has two basic sources:
- The random nature of the distribution of photons within light itself
- Electrical current in the camera's electronics that is falsely measured as signal
The latter is generally constant over a given time interval for the same sensor operating at the same temperature. If the sensor gets warmer, it also tends to be noisier due to this "dark current".
The former is due to the way photons aren't uniformly distributed in light from light sources. If the intensity is very weak, the randomness is more noticeable in short exposure times, and the image will be noisier. If intensity of light is very weak, a longer exposure that collects more light will average out some of the randomness of the weak signal. If the intensity is strong, the larger number of photons streaming from the light source will average the randomness out more quickly.
The amount of noise due to this randomness within light is known as Poisson distribution noise, after the name of the mathematician who first introduced the concept of discrete probability distribution that may be observed in a wide variety of applications. Poisson distribution noise increases at the square root of the intensity of the light field. We also often refer to Poisson distribution noise as "shot" noise. So as the intensity of light increases, so does the shot noise! But since the intensity increases as the square of the randomness, the overall result is that the shot noise gets more and more drowned out by the sheer amount of light as the amount of light increases.
So what makes an image noisy?
Not enough light collected over too short an exposure through too narrow an opening.
If you shot both events at the same apertures and more or less the same exposure times, then the difference was that the venue for one event had more light on your subjects than the other. Most of the light in the second image is concentrated at the spotlight in the field of view. Everything else is several stops darker.
