This type of monitor uses a cathode ray picture tube. This is a design taken from analog TV technology. The image on the screen is glowing pixels of red, green and blue phosphorus. In the neck of the tube is an electron gun. It fires elections aimed at the inside center of the tube. The exterior of the neck of the tube is wrapped with electromagnets. These cause the path of the electrons fired by the election gun to swerve. The magnets cause the electrons to hit the tube at the upper right corner. The electromagnets force the electron hits to move from right to left tracing out a line like a like on a typewriter. The intensity of the electron hits dictate how bright the spots of phosphorus will glow. Again the hits are moving along a line right-to-left. The glowing phosphorus persists in glowing for a time. After a line is complete the electromagnets cause the electron beam to drop down to the next line. Again this line is targeted (scanned).
TV and some monitors paint every other line. That would be line 1, then line 3, then line 5, then line 7 etc. When the scan reaches the bottom it is directed to scan line 2, then line 4, then line 6, then line 8 etc. This is called interlace scanning. This was the normal in analog TV and monitor displays. The scan rate is locked on to the cycles of the AC main power grid, 60 cycles per second in North America or 50 cycles in Europe. Interlace scanning paints every odd line completing the task in 1/60 or 1/50 of a second. Then the even lines are scanned. The complete picture takes 1/30 of a second in North America and 1/25 of a second in Europe. More modern transmissions use progressive scanning; this method scans like a typewriter types. The image quality is doubled.
Keep in mind that the glowing phosphorus does persist, however when a picture of the screen is taken you are seeing different intensities depending on how many milliseconds ago that portion of the screen was painted. Your images show scan lines with different shutter speed times. Thus what is bright and what is dim is dependent on the persistence time of the phosphorous. Your image showing lines is normal for a screen of this type. You used different shutter speeds as you changed the ISO.