Hello @SteveAllen,
Sharing a couple of inventory concepts from our platform that will help you with the math
The on_hand is the total sellable units in your possession.
The available quantity is the on_hand minus the already allocated units to pending orders.
When you pick up those orders, inventory is moved out from the bins and into the totes. You can check this quantity in the in_tote field.
The sum of the quantity in bins with sellable = true plus the quantity in_tote should amount to the same quantity as on_hand
Let me know if you have other doubts.
Have a great day!