How does the allowAutoUpdates setting differentiate? I'm not quite sure from the documentation what the difference is between 'minor' and 'build' updates.

How would the allowAutoUpdates general config setting have treated the update from 2.4.2726 to 2.5.2750?

What would happen when the 2.5.x update was released, depending on whether that setting was true, minor or build?
Craft's version numbers follow this pattern:

major.minor.build

So your update was a minor update and you could have blocked it with the settings build-only or false.