They are pretty predictable, the average low temp in August is much higher than the average high temp in January for example. OT getting made bigger was hardly uncontrollable, they could have capped the number of tickets and not allowed MU to sell more than that number until the following season.
Then you've got transfer windows, not really the same conditions if the opponent has made transfers between fixtures, e.g. last season depending on fixtures you might play Arsenal before or after Sanchez left, that's a conscious decision by the regulating body to permit mid-season transfers.
That said, if the rule is in place it should be adhered to, I guess it was a bit of a weird scenario because the FA have a vested interest in Wembley rather than it just being some random ground Spurs were using.