It's true, not all people moving from BC or Ontario are automatically liberal. We tend to forget there are still large amounts of conservatives in those provinces, just not a large enough amount to win federal elections. As you said, its more likely for a conservative or politically ambivalent person to move here than it is for someone who's solidly left wing.
Forgot to mention, the top line number also skews it. The Liberals actually polled behind the Conservatives in the 35-55 age group, and I think that's the primary group of people moving from BC/ON that are more dispersed to affect elections. Unlikely a 65+ Liberal voter is selling their multi-million dollar home to move to Calgary. Younger people moving for job opportunities are more likely to congregate in downtown/University area which only affects a few ridings.