Canadian 28" x 1-1/2" (and 28 x 1-3/4") is 700C equivalent (i.e. 622mm BSD). However, Canadian 28 x 1-1/4" uses a larger bead diameter to maintain the 28" nominal outer diameter with the smaller section width. This results in a 630mm BSD, the same as 27" x 1-1/4" tyres.

When you see 28" Canadian you automatically, think 700C equivalent, as it was by far the most popular size. It's easy not to pick up on the 1-1/2" vs. 1-1/4" distinction. I've done it myself, more than once.