GVW or MTPLM?

Everywhere one reads about the MTPLM of caravans and GVW of cars. I ask myself why this differentiation? The two terms are totally interchangeable, so why not be consistent and stick to one or the other? Things aren't made easier by the fact that the law talks about GVW (yes, even for trailers, although the Directgov website, for some reason best known to whoever wrote the text, introduces another term, MAM, Maximum Authorised Mass) but amendments to the law refer to MTPLM (also for cars). It's about time this confusion ended.