Sloppy
The most important thing to say about this is that the flight crew were sloppy right from the start. They were not ever told to climb to FL020. It was FL200. Add two zeros ....that's 20,000 feet on 1013. It doesn't matter where you're from or where you do most of your flying or whether the controller said "flight level two hundred" or "flight level two zero zero". If the clearance was written down after being read back properly, what's the problem for a commercial pilot? So it was just sloppy, end of story.