Now you see, that actually makes sense to me, compared to the OP basically using a Mandel like a standard expander ball after over sizing the neck.Once you squeeze the neck from the outside that pushes any variance to the ID of the case. If you run a mandrel it makes the ID of the case properly round and any of the inconsistencies will be pushed outside and not have affect on the projectile.
To select a bushing I take a loaded round and measure the OD and -.003" then I run a mandrel .002" under bullet diameter.
Now in theory with a turned neck there shouldn't be any inconsistencies in or out but if you are using unturned brass, keeping the variance on the outside is good. Sort of the same idea as Lee collet dies and why they are so effective.