Getting different serials is normal since I'm assuming you didn't order them all at the same time. Also, 12 and 13 keys are different, so even if you ordered them at the same time your "downgrade" key for 12 would be expected to be different.
I don't think you're doing anything wrong, 11 licenses is not a vast amount as most volume licenses are for 5 users and up. I would keep a log of which machine/user has which key installed, it will make it easier to track.
So when purchasing a new soctware I should use the last contract number received?
Sent from my iPhone
You should really be contacting a filemaker sales rep to have them answer any licensing questions you have:
You should not need to reference any prior contract numbers if you're purchasing new software. Renewals and upgrades offerings require that.
AVLA's and VLA's are only available for 5 or more licenses.