At some point you just have to embrace the ambiguity and take the numbers for what they are. For me I just pay attention to the same reading each time - normally the barometric altimeter on my 305. That having been said, at the end of a ride I did with a friend yesterday the barometric altimeter on my 305 had registered 4200ft elevation while my friends' 705 (also with a barometric altimeter) read 3800ft, I'd looped back once for maybe 50ft more gain, but other than that we'd ridden the exact same route.