ems are relative to their parent so li is relative to nav and not nav's parent. You reset the font size on the li. Assuming it's different than what is on nav, that would also change the value of an em on the li to 1em=14px.
If the font size for the navlist was never changed, and equal to the default size of the body which is 16px, then that's your problem cause 1em on navlist would equal 16px while 1em on the li would equal 14px.