Why? What have you done? Sh#t yaself?

(surely I'm not the only ex-pat who can't help but think this every time an American says 'excuse me')

No more/less silly than saying "sorry" or "all right?" all the time - every language has its verbal social fabric placeholders!

I thought saying "excuse me" was just good manners? What should we say instead?

Rene

Do Americans use 'excuse me' differently to Brits?

I say 'excuse me' if somebody is blocking my way, has their supermarket trolley in my way, if I need to pass somebody etc. Much better than just sticking out my elbows and shoving everybody out of my way.

Me too

Nah, we say it at those times as well.

My Canadian hubby is for ever saying "I'm sorry" and I'm forever asking me "What for?"

I have seen many English school books for foreign students that have a small chapter on when to say 'excuse me' and when to say 'I'm sorry'.
The general consensus is that 'sorry' means you need to apologise for something and 'excuse me' means you need to grab somebody's attention.
I say 'general' very lightly. There are a million expressions with both of these phrases.

Yes, there is also "pardon me" .

I always say excuse me if I want to interrupt a conversation or if I need to squeeze past someone or reach across. I also use it sternly if my dude says something naughty or "I beg your pardon" which I could never understand because why would I be begging for his pardon if he said something he shouldn't?

I will occasionally reply with "Why, did you fart?" if my son or wife says it, but it's not out of the ordinary to hear it, especially if I'm in the way as I often am

They definitely use it differently. Well out here in the Midwest they do. It's kind of a replacement for a grunt of 'sorry mate' but they say it very clearly. They use it for way more than just for asking politely for someone to move.

It's not a bad thing (they don't mean any harm - far from it) but I still can't stop myself from thinking 'why - what have you done...'

Absolutely! Perhaps heard a little less these days except maybe when one says 'pardon' instead of 'what' when you need somebody to repeat what they said.
One of my Mother's pet hates (and she had many), was saying 'what?'

But if they said "I'm sorry" wouldn't you still think, 'Why? What have you done to be sorry for?'

No I wouldn't because that is was I was brought up to hear back in England. I wouldn't think anything of it.

I go to the grocery store here in Indianapolis and I'll hear at least 10 "excuse me" and there really is no need for it .(it makes me giggle too) .. go to the gas station and they cut in front of you or steal your parking spot when you've signalled ... not one single "excuse me" then just the finger!!