I'm in the US and I have to support this analysis. I dated someone from Canada when I was younger, and at first he expressed the same thought as the poster above - he didn't hear the difference, so he didn't get it.

Until he came to visit me in the states. The difference between how his dad and my dad talked was so apparent, the stereotype just "clicked." To say "aboot" is definitely an exaggeration, but in all fairness, it's really hard to repeat a sound that isn't in your native dialect. There is no US-English equivalent for the "ou" sound in Canadian English "about."

It's like when a Japanese speaker uses a Z sound instead of English TH - it's not because Z is exactly what they hear when an English person says TH, but because the TH sound doesn't exist in Japanese at all. In both cases (and so many more across the world), speakers substitute non-standard sounds with ones they already know.