There's been a slight odd situation with Hugo Award nominations that I've called the "Canadian effect," whereby Worldcons either in Canada or immediately after seem to have a surge in nominations. Look at 1994 (Winnipeg), for instance, which had a big spike. (Its predecesor was ConFrancisco, one of the larger Worldcons, which meant there was a larger-than-usual nominator pool.) Toronto (2003) and Montreal (2009) also had large turnouts, and now this year, following a Canadian Worldcon, we have a new record high turnout. Yet the per-country chart shows that Canadians make up on a small proportion of the nominators this year. As one would expect, Americans dominate voting. They almost always do (probably; see comments below), and Americans typically make up the largest single group (if not always the majority) of the Worldcon's members.
I don't know if this "Canadian Effect" is just a cooincidence, or if there is something about holding a Worldcon in Canada that energizes the potential electorate in some way.
Edit, 13:15: The "almost" may actually understate American domination of the nominations process. I've added additional weasel words above in light of comments below. I don't think we'll ever know for sure, as the data needed to prove it is unlikely to ever be available. I'd be surprised if much of the data still exists at all.