picksplit algorithm

In rtree_gist.c
postgresql version 8.0.9

maxoff = entryvec->n - 1;
nbytes = (maxoff + 2) * sizeof(OffsetNumber);
listL = (OffsetNumber *) palloc(nbytes);

why the maximun offset is set to entryvec->n-1 ?

why nbytes is set to maxoff + 2 ?

I hope you can help me
regards
jorge