I've recently bought Nexus 5x from amazon and it came with android 7.0.0, but official specification says that it comes with android 6. So question: does it mean something wrong with my device (used, refurbished etc)?
There are several ways for testing whether an Android phone is refurbished. How to tell if your phone is refurbished gives a list that you can try, to begin with. It is not guaranteed to work, but it is worth trying.