House leadership doesn't want to make their guys vote on the bill
again, so they certify the Senate bill is the same as the House bill,
and send it to Bush.
Bush signs the Senate bill into law.
Edit: Hmm, it's not clear the change was significant enough to actually warrant a revote... I don't know:
As the measure was being sent to the House last month, a Senate clerk
inadvertently changed that 13-month-restriction to 36 months, a $2
billion alteration. With the mistaken change, the measure squeaked
through the House, 216 to 214.