It's an estimate of how many losses I'll have in a week total if run rank 4 at every opportunity (21x). 10% would be 2 losses. This factors in that I run a Wind team against Thunder on Mondays and that's likely a guaranteed win. I don't have enough data points to say for sure, but I ran my Water team 6x on non-Fire Rank 4s so far (Sunday and Wednesday), and I have 1 loss (which I used an elixir to recover from since Water). I also don't have the spawn materials to run Rank 4 at every opportunity, which limits data points in the short-term.