Looking at the pic (
SWA1380 - diversion to KPHL after engine event) of the window it appears that the piece of shrapnel did not penetrate the outer pane but came in at an angle, puncturing the skin adhacent to the window frame and then shattering the inner pane.
Edit: in light of later reports and photos this is clearly wrong, the outer pane is indeed missing (it appears to be intact due to the quality of the photo).