You have an item that is marked for free freight (Item Edit>Logistics tab). However, when you add that item to a cart that has other items, the total freight price drops. Since the item you added has free freight, you expected the freight price to stay the same.
What actually happens is the percentage of the free item's freight weight of the total weight is removed from the total freight price.
The following example used actual UPS ratings and includes a 1 lb. package weight added to the total weight.
item1 = 2 lbs.
item2 = 1 lb. and has free freight
Add item1 to cart. Freight = $16.07
Add item2 to cart. Freight changes to $12.74
The total weight for the order = 4 lbs. (includes 1 lb. package weight)
If item2 was not free freight, the total freight price would be 16.98.
Item2 is 1/4 of the total package weight and therefore 1/4 of the freight price is free.
1/4 off of $16.98 = $12.74