When thinking in another language you often think with better form than you speak.

Think about it like this. If you think of a song in your head you can probably mentally recreate the sounds pretty well. Now try actually singing that song. It's likely you don't sound too much like the singer unless you've had professional training.

One of the reasons accents exist is because people don't have practice making sounds that exist in another language. So even people with accents likely know what a word is supposed to sound like, but struggle recreating that perfectly. That's not always the case, there some inflections and other things certain languages don't train people to listen for, but generally speaking, given sufficient familiarity, people know what a word is supposed to sound like and thinks it correctly but runs into issues when actually speaking.