I can’t say I’ve always wondered about this, but I must acknowledge the truth of the matter:
It has been noted that “Uh”s and/or “Uhm”s in linguistic, cultural and situational settings can perform varying functions not only according to their timing and their intonation, but also dependent on their position in a sentence.
Curiously, I tend to use “um” here and “uh” in fiction. I’m not quite sure why I make the distinction.
(Via the Local Malcontent.)