JohnCW A lot of great info here - @JohnCW is totally correct in saying that the original artist almost never produces the sheet music for their song. Even for more academic music (e.g. classical) the composer's score isn't going to be what is sold commerically. There are always substantial edits required.
Often, as John says, that is transferring the song from guitar->piano or similar. Sometimes it's about reducing needless complexity (does that exact improvised lick need to be captured, or is it more important to get the main "feel" of the song across?). Sometimes it's about putting it in a more standard key.
It is definitely possible to make exact/precise transcriptions, if that's what you need (as many of our clients do). But commercially release sheet music is extremely rarely that precise.