Nothing is wrong.. Nicehash is the marketplace.. get that! They need a person who will BUY your hashing power.. and if his/her highest bid is everyday lower.. you will get lower reward... easy as that...
You are not mining BTC or anyother coin on nicehash.. you are selling your hashing power to the highest bidder..
Is this why NiceHash sometimes switches to algos that are clearly not the most profitable at the moment? For example, I am doing a comparison between it and Nemosminer on MiningPoolHub with both getting a single GTX 1080 and I just caught NH switching from Equihash - which was projected to earn $4.31 per day - to Nist5 - which caused projected daily earnings to plummet to $2.10. Based on what you wrote my theory is that there wasn't enough Nist5 hashpower available to fill outstanding contracts so miners got switched to the algo even though it wasn't even close to being the most profitable.