Xkeeper | SMB1 gravity change demonstration @Xkeeper0 | Uploaded November 2008 | Updated October 2024, 2 hours ago.
Gravity changes when jumping.

Every time you enter a new area, gravity lowers. Mario bounces higher off of enemies, falls slower, that kind of thing.

However, it reverts to "normal" gravity as soon as you jump.

I suspect this was done for the "Fall into the level" code, so it didn't look like Mario fell that fast... if you have a way to go up without hitting the jump buttion, well...