Maybe, maybe not. I've looked at EK fares to India, ex-US and YYZ, and have never found them as the cheapest option, and never even close to it. Maybe its my routing. ex-US I've certainly seen QR cheaper. But never EK.

On the other hand, don't think the OPs situation is really any different than the broader debate on fare differences at border airports. Yes, leaving from SEA instead of YVR, or BUF instead of YYZ, to just about anywhere (except Canadian destinations, and around specific holidays like US thanksgiving) is going to be cheaper. Its been like that forever, and likely not going to change anytime soon.

However, to do a fair comparison with the itin leaving from SEA instead, would probably be best to actually compare prices SEA - destination and then take the ferry down. I bet AC's price drops considerably if starting from SEA instead of YVR, and especially YYJ.