This is strictly speculation (since at&t chooses to be so tight lipped about MicroCell issues) but what you're seeing might be the result of a firmware or back end update.

There was some talk not too long ago claiming that a change was propagated and that change effected the way a phone recognized it was in MicroCell coverage.  The idea was supposedly to reduce the tendency of jumping from a MicroCell to a macrocell and thus dropping a call.

About the time those changes were supposed to happen I noticed an improvement.  At my location we have some macro coverage but it's not great.  The result was when I first got a MicroCell I often had calls handout to a macrocell and then drop.  These days that rarely happens.  I can't remember the last dropped call.

Again....I'm just speculating/guessing, but if I'm right, the trade off of having to wait a little longer to see the MicroCell when you come in to range but having better reliability once in range is worth while to me.