I don't think that you can necessarily take that for granted. There is a valid Z fare for the other flight combination. But there may be some reason why it isn't valid for the combination that you'd like - flight restrictions, DIF considerations etc. It's not necessarily the case that a fare either is or isn't valid for all flights on a particular day. Nor is it necessarily the case that if you can see seats in a particular booking class, they are bookable using the fare that you'd like to use.

At any rate, as I said above, when I looked yesterday ITA pricing was actually accurately matching AC pricing on those two sets of inbound flights, so your primary sources are not being misleading.