I'm having similar issues with some of my Gigabyte 1060s.
First I want to say that they don't come with a predetermined type of memory, some of them come with Hynix and some come with Samsung (the better of the two).
I have 2 full rigs right now, some with ASUS 1060s and some with the Gigabyte.
The gigabyte perform better in general and seem to have a higher chance to come with the Samsung memory module and thus clocking higher.
When fully overclocked I seem to be getting around 22-24.5 per Gigabyte card.
Now on to the issue:
The two full rigs are running windows but the third one I'm working on is running simplemining OS.
Currently has 2 of the Gigabyte cards in it and they are fully overclocked but they are only hashing at a rate of around 20, so 2-2.4 lower than my windows equivalent.
Using the same miner, same overclock but different OS and motherboard.... No idea whats causing this drop in hashrate