At best the Yes vote was weakly pro by numbers in the city. Nothing like overwhelming.
Probably, very strong influence of the fishing industry, both from sentiment and as a lobby: the EU would have permitted non IS boats into their economic zone, and there would be concerns in the fisher industry community at large about that economically.
I think the city thing reflects Youth attitudes to pan-europeanism.
Also, "No" just had to say "but our fishing zone" so like "take back control" in Brexit, very simple message. "Yes" had to soft sell positives which were at best nebulous. Unlike brexit I think this has less downsides. The future may change that but for now, I think it won't alter much given that Iceland is already in NATO, already in Schengen, already has security guarantees, has full energy independence. As a complete outsider who has visited recently, I would have loved it to be in the EU but thats an externally driven sentiment view.