Yeah, Michael, that's very common, and it's one reason almost nobody here (or seemingly anywhere outside a doctor's office) pays any attention to formula-based estimates of your personal max. It's hard to measure, as well, and some think it's not safe to measure it. I'm not a doc, so I don't know.
What I as well as a lot of others have used is the ventilatory threshold, which approximates the lactate threshold. Friel's book, referenced here by Allegheny as well, contains a procedure for that, that one can do oneself. He also has a method for defining your training zones based on it, and an excellent explanation of what the zones mean, how to use them, and the underlying theory behind such training strategies.