If you have 5 cards, and then edit them to 4. You get the following warning when you click the red exclamation mark.

"To indicate which of the 4 cards you are trading, check the boxes in the last column."

You are marking which of the *5* cards you are trading away. I'm guessing this isn't a simple fix of adding +1 to the displayed int, because if multiple cards are deleted, the count will be off by that number.

The way I reduce the number of cards is by editing the green column on the far left, while in inventory view.