I didn't notice the helmet-to-helmet contact when the play occurred, nor did I notice it on the replay. I didn't see it until I heard that it happened, then saw the replay again. It was not an obvious infraction. The one at the end of the playoff loss in Arizona happened amid the chaos of a fumble, so that was not an obvious one either.
The refs are on a hair trigger about making that helmet to helmet call, so I really don't think they intentionally overlooked it in either case.