Thinking about this all from my wife's perspective, and trying to put her reaction into context, here is what I have come up with....
She stated that she did not notice how much I was drinking or how negatively it was affecting me. I am partly to blame for this because I worked hard to make it seem like it wasn't a problem, and I demonstrated an uncanny ability to drink and drink and drink and not turn into a catatonic A-hole w/legal troubles and penchant for kicking out side-view mirrors on cars. Furthermore I never confided in her the feelings I was having about it.
She sees her brother as the obvious example of what a problem drinker looks like and behaves like. Compared to him and concerned for him, I can see how she might not have noticed what was going on with me.
This all came "out of the blue" to her, like one day I decided all by myself that I was having a problem and then just quit drinking, but never discussed it in the past with her.
And finally, she must think, "How much of a problem could he really have if I didn't even think he had one and then one day he just quit drinking and never looked back?" My present and future success with this might seem to her like I must have really never had a problem if I made quitting look that easy, particularly in the light of her brothers ongoing and very problematic alcoholism.