[jpthomas]

To my untrained eye, in the second image the piece of insulation appears to be about to impact the underside of the shuttle wing in an edge-on configuration. If this is the case, the energy of the impact might have been concentrated over a fairly small surface area of the wing. The potential for significant localized damage would seem to be much greater with an edge-on impact. I wonder what assumptions the NASA engineers made regarding the orientation of the insulation at the impact point when they simulated its effect on the tiles. I'm sure they'll be looking at this more carefully.

[Wolfstar]

To my best understanding, NASA is looking at the possibility that the impact may have damaged the wheel-well bay door seals in some way. So your layman's understanding may prove to be correct in the long run.