I use Priority Mail domestically and First Class Mail typically for international orders, so my Canada rates are usually lower than the rates to the US (I am shipping from the US). Perhaps the system noticed that my rates to Canada were lower, and assumed I was entering them in the wrong field, assuming the lowest shipping rates would be domestic, and automatically corrected it.
Whatever the case, something changed and it is either a glitch, or something that should be looked into. Had I not noticed, some of the buyers would have been paying too much, others too little.
I'm sure I'm not the only one that this would affect. Just wanted to bring some attention to it.