Q about the built-in Vesa modes

I want to use the native max resolution (1024x768) with my trustworthy old AMW 15’ TFT monitor. Interesting things going on. If I switch to 1024x768x16 or x32 I got a blank screen and the monitor saying “signal over range”. It is obvious that the refresh rate is too high, or at least higher than 75 hz. However if I switch to 1024x768x8 (8 bit color depth) it works well, obviously the 8 bit mode has <75 hz refresh rate so my monitor can display the desktop.

I am wondering why different refresh rates exists for different color depths and/or resolutions. Is it the vesa standard or whatnot? Also for me 800x600 or lower works well with the same monitor. Hm…

So my Q is, will be any change in the future for vesa mode refresh rates? A constant setting like say 75 hz for ALL resolutions/color depths would be nice.