I'm sure we mostly know really, but I'll bite and say that it Its more of a social thing, it doesn't actually mean anything usually, we have just become so used to saying it, but beleive me, people notice if you don't say it.
It never ceases to amaze me the amount of people who just don't 'understand' and end up having a moan about people saying 'sorry'.
I have had to try and explain it to a few foreigners in the past, and one of the examples I give is that, if you are holding a door open for someone, you apologise as you are potentially hurrying that person up to catch the open door, whilst they apologise for inconveniencing you!

You aren't really sorry, and neither are they, it is just part of who we are socially, simples.
Silly really when you think about it, but I think it is just hard wired into us to say 'sorry' at any perceived slight or inconvenience, real or otherwise, as it is almost like an insurance policy to show the other person you 'know'..... And are 'sorry'....
