It wasn't the last category in the context, no.
What I've since found was that using the arrow icons moves the bank/subcategory to the system category without copying it. I was moving the bank by clicking the cog to edit the subcategory and then selecting the desired parent category from the drop-down list. Like I said, for some reason that just copies the bank over instead of moving the original.I'll use the arrows from now on even if it's a bit fiddly. Thank you.