Why does my % achieved towards the next BTC bonus always goes back down after a while???

Is there any proof people are winning BTC's on there?
Anyone won something yet?

Ik heb 3 GTX 1060 6 gb gpu's van hetzelfde merk op één mining rig.
Maar er is één kaart die bijna 15% sneller is dan de andere???
Alhoewel ze hetzelfde zijn overclocked?
Hoe kan dit???

I have 3 GTX 1060 6 gb gpu's of the same brand on one rig.
But there is one of them that mines considerably faster than the other 2, although they are all overclocked the same way...
I don't understand why there is one almost 25% faster than the other 2?