What are so many Canadians duped by this graphic?

This graphic gets several things wrong. First it assumes that Canadian forests are a CO2 sink. But in fact they have been a CO2 source for several years (in other words they are emitting more CO2 than they absorb). The data to support this is publicly available:
https://www.nrcan.gc.ca/forests/report/disturbance/16552


More obvious is the fact the graphic also assumes a misunderstanding of how the carbon cycle and emission reporting works.

The graphic claims that Canada produces 559 Mt of CO2 (the actual number is closer to 700) but the number reported for CO2 emissions is not the total CO2 released in Canada. It is just the amount from post-industrial sources.

Animals (including humans) all release CO2 as part of natural processes. These are not counted in the emissions reporting because they have no affect on global warming.

They have no affect because the CO2 is absorbed by plants (including Canada's boreal forests). During per-industrial times this cycle kept the atmospheric CO2 at levels which kept global temperatures relatively stable compared to the post-industrial period.

So you have to be very careful counting CO2 absorption from forests to offset post-industrial emissions since it was part of the carbon cycle which kept atmospheric levels of CO2 balanced during pre-industrial times.
Perhaps it would be easier to understand this mathematically:
If
A - B = X
and 
C - B = Y
And C > A then Y > X
Where
A = pre industrial CO2 emissions
B = forest CO2 absorption
and C = post industrial CO2 emissions
Yet the graphic seems to coming to the conclusion that Y <  X. Of course this is mathematically impossible.