What the PHRF?

I don't get this. On the basis that I might on a mad whim decide to race Messing About I applied for a handicap. I was told to apply through PHRF MA, which I assumed meant Massachusetts. I was given 226 as my racing PHRF and 240 as my cruising PHRF. Holy crap, I thunk to myself, I could finish a day later than the rest of the non-spinnaker fleet and still win.

I was informed that the MA meant Mid-Atlantic. Ah! That would make sense as I sail in the Mid-Atlantic not New England. I filled out their form with the same information as I had used with Mass. but this time I got a PHRF of 171. What the ..... OK so it's a rating most of the boats I might race against would give parts of their body for but how can there be such a big discrepancy?

Answers on a postcard (or comments?)