It's based on the 1625 strap, since the 1500 strap wasn't stable for me beyond 1875.
That's very strange, I have several rx with hynix and the 1500 strap is the most stable at least up to 2050.
Maybe your cards have better cooling for the RAM? This is an Asus Strix. After tuning the 1625 strap, it was stable at ~27.2Mh with a 1950 mem clock. Now I'm testing 2000.