Yeah that's my thought too. It could also explain why so many tickets have disappeared before the Tuesday (all members) sales all season.
If the Monday sales over the season were all taken up by 15+ from last season, then the 18+ (guaranteed) and 15+ (first ballot) should in theory have left only a handful of tickets for the main ballot.
Instead, there were more tickets available at the end of the 15+ window on Wednesday than I saw at any point this season in the all members sale on a Tuesday, even when I got through immediately because there was no queue to speak of. It was always individual seats and a scramble to get them.
The numbers definitely didn't add up, and as you say it's probably supporter's clubs allocations not being applied to supporter numbers.