A sold out stadium can be half empty if the product on the field is bad. Utah fans could (dunno if they still can) buy their tix year to year, while schools that had a more consistent winning program (or more consistent loyal fans) would have to hold on to their season tix, even when they weren't terribly excited about the product.

Been a while since I spent a lot of time in Utah, but at UW, the 0 and 1 win seasons helped burn through a lot of the season ticket waiting list.

I suspect that ND has a similar wait list for their season tix, so they will be sold out no matter how bad they play, or how fat the coach gets, but that doesn't mean the stands will be full.