Is it? I know that it was Canada who introduced the Rugby style of football to the US via the McGill/Harvard game (after it was introduced to us by the Brits).
But after that, I thought that Canada and the US each continued on their own paths and developed their own Rugby style games, different from each other and different from the original. Sometime in the late 1800s Walter Camp came up with a series of tweaks to US style Rugby, which eventually led to the gridiron game (although it was still different from the current American gridiron rules). Then in the early 1900s, Thrift Burnside from U of T, inspired by Camp's rules, came up with a Canadian version of the game, with some rule differences to make the game more palatable to Canadians. After that, I seem to recall that American football took a few cues from Canadian, but the essential elements of gridiron football that are still with us (line of scrimmage, forward pass, snap to the quarterback, x yards in y downs, etc.) came from Camp's original rules for US football.