I don't think the +1 roll /is/ reused - I think they meant that if you can assume rolls uneaten by the party to which they are served are simply thrown out. Wit

I don't think the +1 roll /is/ reused - I think they meant that if you can assume rolls uneaten by the party to which they are served are simply thrown out. With that assumption, the total rolls "lost" by the restaurant stays at n+1 - compared to serving that extra n rolls (which, if uneaten, may be assumed to have been thrown out as well)