I wanted to update on what I've found out--finally had a chance to talk to someone today.

So ATC in this case includes, from my notes "insurance, service fee" and fuel. Not very specific, and no further breakdown of amounts. No explanation of why it's built that way. This reduces the base fare quite a bit over traditional amounts. E.g., my base fare two years ago was much higher when the ATC was $15.

The fares I was scrutinizing were the cheapest ones I could find for the destination and dates, so by the time you exclude regular taxes (about $150 or more) and the ATC ($320), the base fares were pretty low. For a total fare of $843, only $387 is considered base fare. (Only 45% of the cost is base fare--that's definitely whack.)

So, when I tried to apply my companion voucher for $399 instead of base fare of $387, it said, no, that doesn't make sense, and proceeded to calculate the two fares with out the companion calculation.

What the live agent was able to tell me is that the ATC for the companion ticket was only $65, not $320. I was never able to get that far, to see that. The trick is I have to start with a fare of greater than $399--or she said they could do it for me live, if I couldn't do it online. (We always book our companion tickets live anyway, as between Westjet Dollars and vouchers, and two groups of 2 on the same itinerary--it gets complicated!) So I do save by reducing the ATC from $320 to $65, and depending on the actual fare, I'd save any of the (weirdly low) base fare in excess of $399.

Still with me??

So, with a slightly more expensive first ticket, I do save a couple-hundred bucks on the companion voucher. Looks like all in, it'll cost me about $100 more pp to fly next summer than it did 3 years ago--honestly, not a bad return on my voucher, and probably still cheaper than any other carrier for flights in early summer. (Though I will be watching between now and the time I book, as this arcane weirdness does reduce my loyalty overall.)

And I still have NO IDEA why they have this giant ATC now instead of just a regular fare.

Thanks if you waded through all that. I felt obliged to come back and clarify, as I had been the one to raise the alarm.