Length of movie varies

[alehel]

Didn't really know which subforum to post this in, but I thought it sounded like a newbie question :P.

Is it normal for blu-ray/dvd releases to have a different running time than when it was shown in the theaters, despite not being cut? I've noticed with a few dvds lately that the running time listed on the dvd is often a few min shorter or longer than the time listed on most websites such as imdb and wikipedia. Is it possible that these were originally shown at a different framerate which then had to be sped up or down to fit within the PAL specifications for the dvd/blu-ray release?