Based on my experience with my iphone, I have a suspicion they've optimised somehow for people having fatter fingers. I've noticed that sometimes the auto correct assumes I mean to press a character next to the one I wanted.

This actually would make a lot of sense.

My previous guess was the original version had spaghetti code (ex: lots of hard coding edge cases), and they tried to move it to a more standardized / maintainable code block that hasn't been fully brought up to speed.

The guess was based on a interview video I saw discussing the various prototypes for the original iphone keyboard.

Meanwhile I think Android's has gotten a lot better.

This is interesting, I used Android briefly around 2011-2012 and found text entry to be quite frustrating so if it's improved I'll need to revisit it.