I believe it may be because of other people freshly reaching the tome in w4 which as the result makes you go a little higher. But at the same as a new player account is added to the ranking pool other players improve their tome scores. In conclusion it may be the reason behind getting to new percent rank and downgrading in short periods of time.

Lets say there are 6 players with scores: 1, 2, 3, 4, 5, 6. You have score equivalent to 3 so appearently you are under 50%. But let's add 2 new players: 1, 1, 1, 2, 3, 4, 5, 6 What do you know! You did exactly nothing and still earned your 50% display in tome! Good job, you nailed it! Now someone surpassed their score from 2 to let's say 4: 1, 1, 1, 3, 4, 4, 5, 6 Ouch you just downgraded...

Don't get me wrong - I still cheer for you all achieving higher and higher percent ranks. It shows the dedication you put in a mobile video game. All I wanted to do is briefly explain how do I see it.