It looks like the OP is comparing a 'sale" ticket on Westjet. A regular ticket can often be more $$ than an AC ticket. I'm flying YHM-Halifax in July. I got a sale Westjet ticket for $320ai. The next day my friend booked and paid $450 for the same flight on WJ (sale had ended). Similarly last year a friend arrived from Calgary and the next day his father became quite ill. At 3pm he tried to get a ticket home for that night. WJ was over $500 for the one way fare; AC was $350.
There are many many examples of where WJ is cheaper than AC but the opposite can occur. Plus.. on AC's side they offer a lot more flights to more locations at a variety of days and times..can't say the same for WJ.

.... and the comparison goes on....