Hello, most likely a combination of distance away from your main water line and the the pipe size to the bathrooms. My guess is the one with low water pressure is further away from the largest pipe that enters the home. Also, it may be sized down to 1/2" at that bathroom. 

If the bathroom pipes are accessible from a attic, basement, or crawlspace, then you could make sure they are fed from the largest pipe. 

Also, if your main pressure is below 75 psi, you could increase your pressure in that line and it would increase pressure throughout the home and possibly up the bathroom with the low pressure up to an acceptable range.