There are two main settings for a monitor, the number of pixels, and the color quality (32 bit, 16 bit etc)
The higher these settings are the more power and memory it needs from your graphics card.
If you try to set it too high, and your graphics card cannot cope, then Windows sets it back to a lower setting.
On some computers you can specify how much memory you want your graphics card to use, the more memory the higher the settings.
You can sometimes increase the amount of memory by going into the BIOS.
What computer are you using, and do you know what graphics card you are using?