I didn't swim Butterfly style, why I can see that on my result?

SWim-WiSe.app dipalyes swimming styles you dodnt swim?

Pls refer these 2 reasons can make that : 
1. Apple Watch detects your stroke:
backstroke, breaststroke, butterfly, freestyle.

When you change your style at an idle of a lap stroke mixture of strokes.

While you try to achieve a better swimming technique it might be apple watch won't recognize your style, because it is still not smooth enough.

In that case, swimmer stroke could not be determined.

2. Did you set your Watch correctly 
Make sure you choose the right orienatl settings at this screen on Apple Watch