....and while I sorta-understand refresh rate, it's not something I am 100% familiar with.
I'm in the same boat - I too do not profess to know all the ins and outs of it.
I believe it is a function of what the monitor is capable of, and also the video card. If the video card is capable of a higher rate than the monitor, then the user must use the lower rate - the monitor's.
Did you recently upgrade your video card or monitor, or change their settings?
lachesis: Also, can you describe your monitor for us? Manufacturer name, model number, .... everything that identifies it. With that info, perhaps someone can find the correct settings for you.