I can relate to your story. Many times I controlled my drinking very well, only to have a blow out at a later time.
A couple of reasons come to mind.
First, if eveyone saw me drink only a beer or two they would conclude that I didn't have any problem with alcohol. I did this for a long time, one of the reasons other people never saw me as having a problem.
Second, when I would be in a social situation with others, I knew that nobody in the group wanted to drink like I did. If I wanted to get drunk I would need to do it with a different peer group. For the last several years my peer group was me.
The lyrics from the old George Thorogood song "I drink alone" descibes it pretty well.