If you're talking about the Peter Cushing-movies, I like the explanation given in one of the novels (that I haven't read - I watched the entire tv series and loved it, but also diving into the audio's and novels is a bit too much for me). The idea is that Barbara, after leaving the first Doctor and going back to her life in the 60s, wrote two novels based on her first two Dalek-adventures with the Doctor, changing things around just enough so that people wouldn't think she's a complete lunatic. Her novels were succesful enough to be turned into movies, so, if you take the DW-novels as canon, those Peter Cushing-movies where actually released within the Doctor Who-universe too.
I like that a lot. The (book) sequel to the (book) Forrest Gump does something similar; the first book is made into a film in-universe and the differences between the movie and the book are accounted for as executive meddling.