Nexus 5x came with preinstalled android 7.0.0 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)?
It's most definitely refurbished.  Android 7 doesn't come preinstalled on the Nexus 5X brand new.