Additional units received at FC

I am curious about the process Amazon takes when counting and receiving FBA items. How can there be more items received than I sent? I have my theories, but I'm interested in another opinion. Could it be, there was already inventory there and it was counted in addition to the received units? Did I put an extra item in the box? Doubtful. Items are always sent with the same quantities in the same size box, with the same labels. Additional units would not fit in the carton, and it doesn't make sense to stick an unaccounted-for unit in the box. The product stickers and shipping labels are printed in-house. The machine prints exactly the amount of stickers needed, so when I run out, I know it's all been stickered. Of course, it's counted again before being taped shut.

I packed a shipment on May 29th, "Shipment #391 (05/30/2025). I counted these units exactly". This one, I am certain, had exactly 299 units. Granted, it's in "receiving", no action can be taken until it's been "closed". ASIN B001WMX2IM is a leather-bound, lined desk journal. 34 journals are packed together in one box, but Amazon says there were 35 received.

What causes the fulfillment centers to account for extra units that were not shipped out?

Thank you in advance.