How does the shipping estimator work?

So I'm very new at this, so please bear with me. When I'm running shipping estimator with UPS, it seems to be giving me varying total weights. I've tried to make sense out of it, but I couldn't. Hopefully someone can give me a pointer or two here..

So I have a zencart that's selling some basic items, each weighting the same as 15lbs.

Items Ordered / Box x Weight-per-box

1 item / 1 x 18lb
2 items / 1 x 33lb
3 items / 1 x 48lb

4 items / 2 x 33lb
5 items / 2 x 41.25lb
6 items / 2 x 49.50lb

7 items / 3 x 38.50lb
8 items / 3 x 44lb
9 items / 3 x 49.50lb

10 items / 4 x 41.25lb
11 items / 4 x 45.38lb
12 items / 4 x 49.50lb

It looked like there's a max weight of 50lb per box and the box itself has a weight of 3lbs. But this formula I thought I had cease to apply after the 2nd box. Is it how the estimator supposed to function or was there an error maybe with my installation?

Per Item item Shipping module counts the quantiy of items being shipping ... it is not even looking at weight, just the number of items ...

Thanks Ajeh.

So it sounds like my cart is using a add-on module instead of running on the default install. How do I find out if this is true? (then I may be able to look up additional info for this particular module)

In the admin section, it does show "United Parcel Service" with the green icon and the other shipping methods labeled red.

What do you show in the Modules ... Shipping Modules ... as being installed?

Next to each name is a brief filename like:

Flat Rate flat

Per Unit perweightunit

What do you see installed? What are the filenames following?