Now we are getting into semantics of "seasons." Check out my
episode airdates page at the Simpsons Archive website for examples of what Fox considers being "in a season." Okay, the last time it happened was in Season 16, when THOH XV aired on 11/7/2004, but the "official" Season 16 premiere, All's Fair in Oven Air, aired the following week. Still, if an episode airs on 9/18, most likely it would be treated as "between Seasons 33 and 34."
Another possibility; if 9/11 is a Fox NFL doubleheader day, air the carryover episode immediately after the football coverage - especially if there is another half-hour show Fox wants to promote (for example, the first post-movie Bob's Burgers episode) that can air right after, or even right before, it.
This isn't limited to The Simpsons; back in 2012, Fox aired two new episodes of New Girl on 9/25 - one at the show's regular time slot of 9:00, and one an hour earlier - but the 8:00 one was considered a "bonus" episode while the 9:00 one was the "official" series premiere.