I don't know so much that I'm "mistyped" frequently, as much as it's that I don't fit very well into a single type. Rather, I fit in relatively well with portions of what I read about two different types (INTP and INFP). I'd say that on tests (the validity of which I often doubt anyway), I come up INTP on about 80% of them, INFP on the other 20%. The "function" test, which I *really* doubt (because of the result it gives me as much as anything else), sometimes gives me ENTP.
Reading various descriptions of types, I find that there are many anecdotes and flavors that I can easily sympathize with in both INTP and INFP descriptions - both things that INTP's and INFP's have in common, and some that they don't. If anything, I settled on INTP as I "get" Fe-related thoughts and behaviors, and I don't "get" Fi ones. A minor thing.
Mostly, I think it's just that many type descriptions are collections of behaviors that are frequently associated... but certainly not absolute. They fit some people better than others.