I generally ignore the Overall Score, as it's way too general if you understand any of the individual scores.
The Overall Score is a function of a variety of (fairly) deterministic tests, each of which is quite informative, and most (if not all?) have clear units of measurement. But then they generate a "score" that combines these metrics, with different dimensions. It's kind of like comparing one car to another, by adding their maximum acceleration (m/s/s), the size of their fuel tank (L), their top speed (km/h) and the number of passengers they can carry. Everyone's going to want to weight the different components differently, so the overall score becomes fairly irrelevant.
Use Case Scores
As you say, the Canon 1DX got the same Overall Score as the Nikon D3300, but note some big differences, even just on the summary "Scores" page:
- "Landscape" score (aka Dynamic Range) is 11.8 vs 12.8 EVs (1 stop better on D3300)
- "Sports" score (aka Low-Light ISO) is 2786 vs 1385 ISO (1 stop better on 1DX)
These "Use Case Scores" are already much more specific, and dimensionally sensible, and make far better comparisons.
That said, they are also
- not necessarily easy to understand, and
- not necessarily a useful measurement for all applications
For example, the "Sports / Low-Light ISO" use case is
low-light ISO is the highest ISO setting for a camera that allows it to achieve an SNR of 30dB while keeping a good dynamic range of 9 EVs and a color depth of 18bits.
These chosen values are arbitrary, but the useful part is they're used consistently to measure all the sensors in the same way. This means that while you're only looking at a single value, you're at least able to compare apples to apples. How well do sensor A and sensor B compare, for one particular data point. It's a useful comparison because sensors all tend to perform better at lower ISOs, and all tend to have similar drop-offs as you increase ISO, etc. But you're really relying on that similar performance across all sensors for this to be a useful general purpose comparison.
If you go into the "Measurements" section of the comparison, you'll start to see some more useful comparisons. Lots of data, at lots of different conditions. That's where you can (kind of) start to answer questions like "How much less ISO noise would I get from camera A vs camera B, at ISO6400?". Or, if you already know that you are OK with your current camera up to ISO 1600, then you can use the SNR for your camera at ISO 1600 as a baseline for comparing other cameras (in this case, there'll be a similar amount of image noise in a 1DX at ISO 3200 as there would be in a D3300 at ISO 1600). Well, even that's not quite true, since the SNR data is for 18% grey!
Comparison of performance between complex devices, with many dimensions/degrees of freedom is an inherently very difficult problem. You can often compare individual tests quite well, but the problem is finding generalisable tests that quickly and easily portray relative or absolute performance. I think the "Use Case Scores" achieve this, to a large degree, but only because the technology of most sensors is quite similar, letting you make generalisations like the one above about the 1DX being "one stop better" for noise in low light. (Imagine if sensor noise wasn't a simple function of ISO for all sensors!)
Bear in mind also that one sensor outperforming another isn't necessarily useful. The ISO performance (SNR 18%) of a D3300 and a 1DX is essentially irrelevant when shooting a JPEG of a bright, daylight scene. More useful will be things like the dynamic range (for shadow/highlight detail with harsh shadows). And even then, shooting JPEG, you won't get much more from a larger dynamic range (there's a bit of tone compression to form the JPEG but both are still capable of well over the 8-bit dynamic range of a JPEG). To repeat the car analogy, it's like comparing maximum speed & acceleration between cars for city commuting. You'll never hit the limits of any 'fast' car, so its kind of irrelevant for that application. You really need a good sense of the specific application if you want to compare two sensors in a meaningful way, and that's where you want to know:
- The actual measurements, for comparison (not just overall scores)
- What measurements are useful for the application
- What the limit of usefulness is for a particular measure of performance
The Overall Score is pretty useless, except as a really general guide to someone's weighting of use-cases (not necessarily yours!)s.
The Use Case Scores are much better guides for a few general performance trends between different sensors.
The Measurements let you make your own comparisons, if you know what to compare, and have an application in mind. It's also important to know how much is "enough", or when you get "diminishing returns" for any given application.
Unless you really understand electronics, optics, physics... the absolute numbers are probably pretty meaningless.