Battery problem

I had a really weird situation a few years ago with a Sable that would pull the battery down overnight. It seemed to have an excessive draw with the engine off. After trying several several things myself and a couple of mechanics I took it to the dealer and he put in a new battery, that was all. That didn't sound right to me but it did fix the problem. I did have the presence of mind to ask for the battery that the dealer had taken out and ended up putting it in my F250 with a motor 2 and half times as big. It worked fine in the F250 for several years.

I am still puzzled as to why that battery did the job in vehicle but not another one that should have been easier for it.