File system question - free blocks

Under what circumstances would I see something like this happen (ext3)?

[rohan@box1 ~]$ df

Filesystem         Total Used Free
/dev/partition     10000 9990 0

[rohan@box1 ~]$ rm <some files on that partition>

[rohan@box1 ~]$ df

Filesystem       Total Used Free
/dev/partition   10000 9700 0

So the number of blocks in use went down, but the number free did not
increase. Why?

--Rohan