For most normal humans, if they merely try a different style than what they are used to their performance would probably go down rather than up. After all, you'

For most normal humans, if they merely try a different style than what they are used to their performance would probably go down rather than up. After all, you'd have seen many common positions before and know most of the usual ideas for your chosen openings etc. If someone suddenly plays a different style and also their move quality goes up significantly, that might be an additional indicator of cheating. All cheat detection works in a probabilistic fashion, since it is not allowed (and would be way worse) to actually observe players 24/7 in their home to verify whether they're cheating or not.