We have the same experience Andy, at Brightlingsea where we have to start in a shifty creek, return there for a turning lap marker and with strong tides we find we wither win by up to 10 minutes, or lose by up to 10 minutes usually depending on the wind and tide direction (long beat with the tide = good, long beat against the tide = bad). We race against the B14 National Champion and a few fair to good merlins, ditto fireballs, lasers etc along with some cats who are equally unpredictable.
We still sail off 920 which I think is on the generous side, I did offer to reduce it to 900 but was told that was the race committees decision, not mine, fair enough !
So I think its down to where you sail, winward leeward and we'll be hammered, round the cans maybe not so much, a good advert for individual clubs setting their own PY.
A wise man once told me - don't winge about your PY, just smile and sail to it - wise words indeed