And why would you even think it would work differently?

Do you really that someone can have 6 (maybe even more) Citi AA cards at a time? (If that many, they might not be able to have any other Citi cards, though.) Did you really think that in that case someone would get 60% back (more than half off)? What if they could have 10 Citi AA cards, that would mean they'd get 100% back???

(Citi doesn't put limits on how many AA cards you have, as long as you apply for them "slowly enough". They only put limits on how many total Citi cards you have, but that limit varies from person to person. And I can't rule that someone somewhere might not be able to have as many as 10 Citi cards at a time. And if so, they could 10 Citi AA cards if they really wished. Though unless this worked the way you thought it might, I wouldn't see the benefit of having so many Citi AA cards...)

The way the system works is by "asking" a simple question:

Does the account holder have a qualifying Citi AA card? If so, enable card-related benefits. If not, don't enable card-related benefits. (There's nothing to enable twice, it's simply either enabled or disabled, an on/off flag. So it works the same no matter how many qualifying* Citi AA cards you have.)

*There's some confusion as to exactly which cards are qualifying, as has been discussed above, because some people with all the "right" wording on their card have been told by Citi CS agents that their card doesn't qualify! Most cards applied for in the past half year are clearly so, but cards applied for many years ago less clearly so. And we were told that qualifying cards would have their annual fee raised to $95, yet the Citi retention reports thread shows most people still getting $85 annual fees today!