Well, disregard this post....
Just recalibrated in distilled water and both are reading at 1.003, while they read 1.004 in tap water. I didn't think that hydrometers could shift out of whack, but perhaps I screwed up when I checked them in distilled water months ago
They also read 1.085 in 1.083 wort.....So rather than doing some tricky math every time I'm just going to take the average and subtract .003 from all measurements.
Sorry for wasting everybody's time.