Could it be that the distance between the top and bottom 20% is less in Canada, making it easier to move from one to the other?
Average earnings of Canada's 1% - $358,238 USD - http://globalnews.ca/news/3079173/heres-how-much-canadas-top...
Average earnings of US's 1% - $1,153,298 USD - http://www.mlive.com/news/index.ssf/2016/08/see_the_average_...
Couldn't find averages for the poorest but Canada's bottom 20% is 0-13,223 USD - http://www.moneysense.ca/save/financial-planning/the-all-can...
America's bottom 20% 0-18,500 USD (US Census Bureau, 2006; income statistics for the year 2005)
Looking at raw numbers skips over a lot of things such as healthcare and education. You're probably better off being poor in Canada and rich in the US.
You're just describing the results of inequality: two things are further apart than two other things. It doesn't tell you anything about the cause of inequality, only the magnitude of one symptom.
It's like saying "a mile is longer than a kilometer". Well, yes that's an apt description but it doesn't tell you why that is the case.