Extent Allocation/Free Space - SQL2000??

I've faced with this problem in my DB:

There are some tables that suffer inserts and deletes daily. These tables have 3 nonclustered indexes including the pk.

I perceived that the space used for these tables are growing day after day even ocurring daily deletes.

These inserts and deletes follow the keys of the pk in ascent order.

I ran DBCC SHOWCONTIG on the tables and got results like this:

- Avg. Bytes Free per Page.....................: 7996.3

When I transform the pk to clustered this problem doesn't happen.

You realized the consequence of this: the users complains because the DB is without space, but it's not true!

Anybody could help me to understand why the extents are not being deallocated?

Thank's for help!