Yeah, it's certainly not an easy metric to measure. Though, i suppose that relatively speaking, comparing what you got year to year with someone (that is, comparing what say I got back in 2002 to what someone here got in say the last couple of years or so) isn't exactly proportionally correct, but at the same time is probably mostly accurate in the sense that, law of averages and all, the scores probably don't fluctuate that greatly year to year. That is, of course, barring some sort of extreme increase to the overall intelligence of future test takers.
Though, it is interesting to wonder. My 31, and your 30, how do they REALLY compare up? Would my 31 be like a 28 or something for your year? Would your 30 end up being like 33 or 34 back when i took it?