You are correct. I think OP was asking why trade throughs happened, so I was thinking in terms of liquidity takers. The hidden orders which improve on the NBBO are implicitly protected because there's no reason for exchange to route the crossing order out when they can match it internally. So for passive orders you don't need to be concerned ... You will not be traded through. For aggressive orders, you need to always be concerned as you maybe chasing stale NBBO.