ZBB: Why Not Counting Bugs in the Last 48 Hours?

Eric Lippert an SDE somewhere in the VS05 effort describes ZBB:

“Zero Bug Bounce (ZBB) day is the day on which, even for a single instant, every bug in the database is either (a) postponable to the next release/version, (b) fresh — discovered within the last 48 hours, or (c) not fixable right now.”

My question is, why not count the bugs in the last 48 hours? Are those somehow less valid than the bugs found 49 hours ago? And what’s with 48 hours itself? Why not 36? Or 72? Pathetic product team wannabes wanna know!