Micron, SK Hynix and Samsung make memory, each brand makes something slightly different and some can achieve more than others when overclocked. If you want to invest thousands of dollars into a rework station and want to source Samsung GDDR5 to put onto your GPU, you can, but you are not, but it is possible. Honestly, it doesn't matter that much, is it really going to effect your performance.
Also you probably could have found out the RAM manufacture before you bought the GPU, manufactures don't want to add multiple variables if they don't have to, it adds more places of failure so more not-valid RMAs happen. MSI released a GTX 1070 in China that had a 100% RMA rate since China was the only country that they sold that card with that particular memory, so no one needed to fuss about and hoping the don't have the "BAD GPU".