Yeah I don't go by the tests much for similar reasons to above. However, I sometimes 'take stock' of how high I think my preferences are at a given time, and they do shift around. Sometimes I feel like I'm only just scraping an E when I'm tired and stuff, and just feel like being alone at home for a while. Also sometimes I feel more J than P, particularly when I'm around my brother, whose total flakeness tends to force me into taking the J role to stop things from falling to pieces. Even N and T, my two most stable preferences, get stronger and weaker, but to a much lesser extent.
There's also the question of what the percentage actually means. Is it that the 50% mark is the 'centre' and going above it or below it it what gives you the letter, like say if you're looking at the T scale, under 50% preference would make you an F, and over a T, giving you only 50 points to play with, or is it that you take it that T is the preference, and the percentage is to say how strongly you prefer it over F?