Also an official measurer here... A measurer goes through a LONG 2-3 day course to become certified. He/she also has a very detailed manual that can be refered to in the case of nearly every imaginable horn or antler configuration. And, every two years, that measurer must take a test to re-certify. Lots of people say they can score game and know how to measure, but if the guy is an OFFICIAL P&Y measurer you can be assured it will be done right. And like bowuntr said, any differences would be minimal, if at all. In the case of pending record animals, it will also be scored by a committee, or panel, to ensure consistency.
Additionally, as a hunter, score shopping is frowned upon. A certified measurer is not really supposed to measure an animal that has already been scored by another P&Y measurer. Not saying you would, but forget about hauling your animal around to every measurer in your area to see who gives you the best score.