The boot process in windows is done using a generic frame buffer "driver
" which is guaranteed to work on all video cards before the real video card driver boots up. So yes, the boot screen will be low resolution till further along in the boot process where is should go back to 720P or whatever you have your video setting set at at the login screen (or desktop if you have no login).
I think I've seen the boot screen at a higher resolution but probably nothing over 800x600 or 1024x768. It would default to 640x480 though unless it KNEW that your monitor could handle more.
So if it's low rez only during boot but then normal when at desktoip then I would say that is expected behavior.