What does it mean to have a low mpv (7.1) but a normal platelet count? (256). My doctor says its normal, but I do have bruising & my bones hurt easily

No worry. Nothing you should worry about. I do not thing that the low mpv has anything to do with your bruising and bony aches. Brusing can be caused by different things like abnormal function of the platelet, problem with skin collagen, thyroid problem, hormonal, etc. If you are bruising easily, you may want to ask your doctor to run some test or refer you to see a hematologist for further work-up.