Jet does some really cool things with their Rules Engine and Basket Building that encourage people to buy multiple units of your products. So they've also built technology around identifying how many units someone is actually buying. By the same token, it's important to send the correct values to Jet with regards to unit quantities.
Let's say you sell a kit that's made up of 4 units of shampoo bottles and you have a title along the lines of "4-Pack of the Best Shampoo Ever!" Jet is able to identify that you have the words 4 pack in the title. Now, if you also send 4 pack as the multipack quantity then Jet assumes it's a 4 pack of the kit, so 16 units total. Similarly, the 4 pack requires a different UPC than the 1 pack.
To make sure Jet accurately calculates the quantities you're selling, be sure to use a multipack quantity of 1 anytime your title contains the word "pack".