Without the recycle bin enabled, when you drop a table, the space associated with the table and its dependent objects is immediately reclaimable (that is, it can be used for other objects).
If the recycle bin is enabled, when you drop a table, then the space associated with the table and its dependent objects is not immediately reclaimable, even though it does appear in DBA_FREE_SPACE. Instead, the dropped objects are referenced in the recycle bin and still belong to their owner.