No, Subscription has nothing to do with this!
I think I partially answered to this question here: Lock to "current release" enabled by default from 7.5 - #13 by davidep
About v6: it depends on RHEL lifecycle. ns6 started from 6.5 up to 6.9, whilst ns7 started from 7.3 and we’re now at 7.5! It corresponds to production phase 1 where upstream pushes more components updates at minor release points.
Furthermore ns6 has no customized kernel modules!