Obviously the difference is that one can and one cannot access Netflix. But how does Netflix ban VPNs while not catching VPNs designed specifically to access Netflix?
The main difference is the question of whether Netflix knows about them yet or not.
In time, the VPNs which can access Netflix today will likely end up being blocked by from accessing the service when Netflix's analysis of incoming connections reveals IP addresses which could belong to VPNs used to circumvent their restrictions.
It is possible that some operators of VPN services may make use of IP addresses which are changed periodically to make detection less likely and this is how they may go for an extended period of time without being blocked.