Calculate feature not working in certain form fields

We use this form as a product receipt for our customers. You enter in the quantity of the item the customer is purchasing then the total cost is automatically calculated and represented at "Total Amount:". The quantity is in the cell to the left of each item.

However, for some reason when the "Night Splint" and "Cam Walker" quantity fields are edited, the Total Amount value does not add those values, quite weirdly though, when you clear the value in a quantity field for Night Splint/Cam Walker the "Total Amount" field updates to the value it should've been before it is cleared! I am not using scripts to calculate this, just the built in calculate feature. I recommed opening the document and looking at the fields "Night Splint" and "Cam Walker" to see what i mean.

Does my question make any sense? I am quite confused by this error so any help is greatly appreciated.