It is possible for a table to be empty, but the tablespace which contains it to be very full, even if there are no other objects in the tablespace.

The reason is that when a table (partition, whatever, it doesn't matter) is created, space in the tablespace is allocated for it. It doesn't matter if you have rows yet or
not. The amount of space allocated is defined by the INITIAL
keyword in your CREATE TABLE sql stmt. When lots of rows
are added and they finally take up all the room that has been allocated, then another chunck of space (called an EXTENT) will be added.

The idea is that Oracle allocates space ahead of time for objects rather than take space as it needs it. This makes it much more likely that an object sits in one place on a disk, rather than scattered all over it.