What do the numbers on the trophys at the thumbnails of the levels mean?

Hello.

when i unlocked a new level, it has got a pretty high number on the trophy-icon of it´s thumbnail.

when i solved it, this number lowers while keys for unlocking the next level are added to the key-counter at the top right corner of the screen.

So it seems like that´s the number of keys stored in that trophy.

But why does it not lower to 0 then at solving the level?

For level 1-4 it´s still 1, for 5-8 it´s 2,for 9-11 it´s 3, for 12-14 it´s 4 and for 15-20 it´s 5.