20% off doesn't really mean 20% off

I have been with AT&T for over ten years (and through several company changes - SunCom, AT&T, Cingular, AT&T). Recently, the company I work for was purchased by Xerox and I found out we're eligible for a 20% discount on our bill. Per the wording on our Xerox benefits page:

"Get up to 20% off your monthly service charges"

I got the discount applied and thought all was well. Never really looked too closely at my bill, but it was around $83, so I assumed I was getting the 20% off my WHOLE bill.

Fast-forward several months and I decide to add my mom to my account. I think it's going to be a good deal since hey, 20% off! So I switch my plan to a mobile share plan and add a line. So, $40 for her line, $40 for my line and we're sharing 4GB of data for $70. Total bill should be $150 with 20% off (so $30 off). 

Long story short, and after a call and chat with AT&T, the 20% discount is only applied to the $70 mobile share plan. They cannot seem to tell me why, but that's just the way it is. They keep saying this is how it's supposed to be. I keep pointing out that the verbiage on my perks site says "Get up to 20% off your monthly service charges" and on my bill, under MONTHLY CHARGES, I see only a $14 discount.

I never would have signed up my mom or switched plans if I thought all I'd be getting was 20% a portion of my bill.

Any ideas? Anyone else have this problem?