Good evening all

I have been shopping for a RT from either BOS or YUL to AMS or BRU in April '15. It appears that flying out of YUL is cheaper than BOS. So here are my 2 questions (which may lead to further based on your answers):

Why is it cheaper to fly out of YUL instead of BOS?

Why is it cheaper to fly YUL - AMS - BRU instead of just YUL - AMS? I understand there are penalties for not completing the trip but if I wanted my final destination to be AMS, why is it cheaper to continue on past my destination and have to drive/train back?

Thanks