A lot of the time it's because the implementations haven't even been agreed between the different browsers. I can't remember any specific examples off the top

A lot of the time it's because the implementations haven't even been agreed between the different browsers. I can't remember any specific examples off the top of my head but I do remember that some of the -webkit and -moz prefixes were ever so slightly different. I think once it made it into the draft most browsers start supporting it using the standard css without any prefix.