There are many times my phone detects I'm holding it horizontally while taking a picture, but I'm actually trying to take a vertical picture, and vice versa.

For those images, the exif information is technically correct, but actually wrong.

EXIF info is often missing or wrong. Photos that have been passed around, going through various services, apps, scanners, screenshots, etc. along the way, frequently have their EXIF info stripped (e.g., for privacy when exported or uploaded), written incorrectly originally (e.g., you scan an old B&W print, put it in sideways to fit in the scanner, and the scanner invisibly includes EXIF data assuming it was correctly oriented), or rewritten incorrectly (e.g., stripped EXIF is replaced with default orientation).

If exif information is incorrect, that's just programmer error at the firmware/library level.

exif is a bit of a swamp, and may well be missing entirely.

I've encountered plenty of pictures with incorrect EXIF orientation, mostly caused by holding the phone at a weird angle.

For example, if I hold the phone parallel with the ground (aimed at the floor), what is the correct EXIF orientation?