"Shared Upgrade" on Legacy/Unlimited Data Plan with 2-year contract is more expensive?

I have a FamilyTalk 700 plan with unlimited data. Today, (December 27, 2015) concludes the 2-year contract on ONE of the phone lines -- the one that is not mine. Mine doesn't end until sometime in March.

I want an iPhone 6s now (not the Plus - still debating between 64 or 128, but probably the 128). I want to pay the subsidized 2-year contract price - SCOFF ALL YOU WANT, but don't act like you don't enjoy paying $400+ below MSRP. KEEP IN MIND, I've never been eligible for a Mobile Share plan, and so I'm not sacrificing some crazy discount each month, because I don't have that service fee to begin with, and from random posts I've read, buying all my phones directly from Apple helped I guess? MOVING ON.

I know the rules on buying a phone at the 2-year contract price - do it online, ship to billing address, cool. MY PROBLEM: When I chose the "shared upgrade" option and selected the phone I wanted, the price was NOT $299 or $399, it was more like $550. I can't say the exact amount because since then I've pressed the "Back" button of my browswer enough times to "undo" the contract switching dates (*because in typical AT&T fashion, you must AGREE to things BEFORE being able to see the fine print, if any!*).

After my "Back" clicking success, I decided to see if the prices were suddenly that much more expensive if my mother (that's the phone line who's upgrade I'm taking) were to use her upgrade for herself, going from her 4s to the iPhone I want. AND THE PRICES WERE NORMAL. By normal, I mean what I expected - $299 for the 64GB, $399 for the 128GB. 

SO WHY ARE THEY $150 (approx.) MORE WHEN I TRY TO USE A SHARED UPGRADE?

There is literally no fine print on this website, and I've googled, and searched the forums, and I cannot find any reason as to why this drastic price difference exists. 

If that's just "the way it is" now I guess I'll have to upgrade her phone to the phone I want, then when it arrives, go into an AT&T store and have them put my 5S on her line and the 6S on mine because I do not trust people over the phone to do the things they say they are doing.