Some sources say that the Hamas militants who took territory and positions have long retreated and the ones currently holding said positions are the second wave, the less trained, less experience and less well equipped. Hamas may have known that an attack on Gaza was likely and planned for their better troops to return ASAP after taking as much as they could. There's been reports that they were sending more of this "second wave" even 30 minutes to an hour ago.
It's important not to trust anything too much, though.