Board Thread:Strategies/@comment-45.20.232.127-20150526184427/@comment-11090687-20150526185432

It has something to do with difference in Stats. Basically it's an indication of the effectiveness (accuracy, total damage etc.) of the selected attack versus an enemy unit. I'm not sure how it works mathematically.

For further reading see the gameplay article with the official explanation by Z2.

Hope this helps! :)