Segment advisor and alter table move partition

After looking at segment advisor findings we planned alter table move partition & rebuild indexes for several table partitions with local indexes

But after some a first run we stop

Segment advisor finding says:

Allocated 18GB Used 16GB Reclaimable 2GB

And after alter table move partition we have 18.2 GB for moved table partition

Move has been perform with no parallel run

Now we stopped to decide whether go on with other partition or not.

Why advisor finding was so wrong ?

Is there a recommended gain percentage to look at before deciding to move table partitions ?