The plugin performs a calculation of what volume the cart items would take up if they were stacked inside a tight box.
So, for example, if you have 1 item of 10x10x10, it will calculate the volume of that item.
Add another item next to it and you now have an overall box 20x10x10.
Add a third item (behind the first) and you have 20x20x10.
A fourth item will not make a difference, as it’s taking up the empty space behind the second item.
Adding the 5th item changes things because we stack it on top of one of those 4 boxes that make up the first level.
Now you have 20x20x20, and adding another 3 boxes to the second level will not make a difference, until you expand the number of columns or rows (which happens after the second level is complete).
That’s why you see a jump in the price.
To understand why UPS is returning different values, I need to see the request and response XMLs that you can get from Admin -> Settings -> eShop Shipping Extension.
Please send them to vinny [at] usestrict [dot] net.