As a college chem professor, the reason for this is nearly always cheating.
"Hmm, you got the right answer with the wrong method, and your friend that you sit next to every day used the right method and got the same exact answer as you, down to the rounding? Haha, what are the odds??!?"
I tried this in university. I knew the right answer but not how to get it, so I made a "mistake" in the calcs. It was circled with "something's not right". No marks.
