Advance fares sort-of do-not-count as they're here today gone tomorrow prices and changes to the allocations can be wildly inflationary for the individual - for fares, the floor for passengers is the buy-at-the-time-of-travel - as that's the one you'll need when life throws something at you.
And they sort-of DO count is people have been habitually using them and find them unavailable.
Withdrawing / reducing substantially the availability of advance fares that have been offered for long enough for them to be come part of people's routine is an easy and perhaps sneaky way of putting up the average fare you expect people to pay over and above other fare rises, without it being straightforward to hang a headline on what might be a substantial rise in the average amount paid.
Yes, it is possible to find a Wednesday to Wednesday round trip, probably at some time of day when few people want to travel, and hail that as "low fares still available" with a hurt look at anyone who suggests such fares are not available. But that's not the point - the point is that potentially that for most people, the cheaper options may no longer be practically available.