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Still, it certainly might happen - the jump you've seen sounds like a very hopeful sign and it may well predict at least an improvement on google.com. Let us know how things progress for you.
For one single competitive keyword, I get these SERPs:
UK - #3
Canada - #3
USA - #7
It's been that way for weeks for the first two; for the US, I've been shuffled between #5 and #8.
If Google scrutinizes competitive keywords more closely, and there's the greatest competition for Google.com searches, it could be a challenge to go higher in Google.com versus the other countries. It probably takes great IBLs from American sites.
I'd be curious to know what percentage of Google users search from the Google of their country. It seems Google automatically defaults to your country and you actually have to change your settings if you're not in the US to get the .com. Don't know how many people either a) know how to do that, or, b) could be bothered.
It is actually quite strange this in itself as the site is very UK, with a majority of UK ibl (sorry slight hijack there)