del_lf_rows or the deleted rows still in leaf blocks, if there are no rows in a leaf block, that leaf block is released and put under free list for leaf block splits.Other wise that block is not released.
del_lf_rows does not count the rows of the leaf blocks for which the whole leaf block becomes empty.

If u see ur stats, lf_rows and br_rows.
lf_rows according to u should have been total leaf rows inserted (minus some br_rows) but it is not so , so total rows allocated for leaf shows some of the leaf blocks did get full empty and thus space reused.


I hope it is clear.
GP