Canadian Coins vs. Ace 150

I recently did a test in which I put Canadian coins into 3in of dirt and then their American counterparts into the same dirt. The american coins consistently rang up "coins" on The Garrett Ace 150 where the Canadian coins (exception of the penny) rang up as iron. Each coin I used was dated 2000+ so the composition should be similar. Has anyone else had problems finding Canadian coins and what have you done to solve it rather than digging up everything that is iron.

Thanks