Actually, the other way around. Lower is faster. I think the scale is based on a theoretical Yacht with a rating of 0. The PHRF rating represent the # of seconds a boat will either be behind (positive #) or ahead (negative #) of the theoretical yacht on a per mile basis.
The ratings change geographically, based on local conditions and the performance of certain classes/boats over time. I don't race, and I'm sure others on this site can provide a much better explanation. I only use it as a reference to a boats relative speed when shopping for our "next boat".
US Sailing PHRF Info
What is PHRF?