It's probably the bandwidth of the video amplifiers and should exceed the maximum horizontal scan rate x maximum horizontal resolution. When you start to exceed the rated pixel rate (bandwidth), the pixels look fuzzy or the brightness to vary between characters in any line. There's not a 1:1 relationship between pixel rate and maximum horizontal resolution, and it's best if the video bandwidth rolls off smoothly rather than unevenly. If it's uneven, you can get moire problems, and one way to reduce them is by intentionally decreasing the bandwidth.