WHY does this happen in Mario 1?

An interesting quirk in Mario 1 that's been noticed for a while by some people, always interested me too, and I decided to find out what's going on.
Thank you, Kosmic, for also being curious about this, which lead me to try and figure it out for once.

[@Kosmicd12]

Nice! Mediafire omega made me aware of the next pieces of the level loading down below, but this visual helped me understand it a lot better. Thanks!

[@IvanFranco120]

I never noticed this oddity before! 😳
It seems the devs accidently used the tree tiles background as solid ground by the firebars, i think nobody did found this before, because they didn't brother to shoot an fireball under the tree platforms.

[@tomyyoung2624]

Yes you're thinking with portals.

[@sanamshams1983]

I experience it too

[@ramiro_echeverria]

Cool

[@nitrofire4482]

Don't really want to be annoying, but what happened to FNaS