The Kroger store you purchased from could be "offline." Most major merchants have a "floor limit" -- a certain amount they are automatically authorized to charge to a card if electronic authorization systems are temporarily unavailable. It used to happen to us all the time when I was in management at Wal-Mart about 11 years ago. Back then, the floor limit was $100 for each of the major cards. Anything more than that and the POS would kick back "Floor Limit Exceeded" and we would need to call in for a manual authorization.
I believe I remember reading that AMEX merchant accounts usually have a $0 floor limit, but I would imagine that larger companies such as Kroger may have different privileges. Despite being in front end management with Kroger for a few years, I have never needed to know what that limit might be. Either it's high enough that even our biggest spenders weren't hitting it, or the systems are now reliable enough that it's never an issue. The small company I work for currently has $0 floor limit on all four card networks.
Aside from the possibility of an offline authorization, it could just be a quirk on the AMEX web site. That seems to happen a lot!
