Why does it shock me every time?

So, Davis has been excited to go back to school-probably since the first day of summer break.  This is a good sign.  He loves school now that we have him at a school that gets him. He is "normal" at this school; and even when he's not, the other kids all get it.  One thing I have raved about is that he looks like a typical child when people see him in this context.  He has been chosen to be an ambassador to students that come and visit.  He has friends and they like him-even with all his Davis-ness, because they have quirks, too.
Today was meet the teacher.  I love that they do it on Friday and don't start school until Wednesday.  I'm thinking they know their clientele need a little additional time to process the change of the new year.  He couldn't wait to go, and was rather excited that he didn't have to wear his uniform.  He talked about all the people that we would see, and tried to remember the name of his new teacher.
We show up and were directed to room 12.  Uh-oh!  We had been preparing him for room 15 all summer.  He was in room 12 last year.  All the classrooms at his school are multi-leveled, but it's all about precedence for him.  He spent one year in one class room, one year in the next classroom, and fully anticipated to moving up to the next classroom this year.
Here's the problem...all of his friends did-with the exception of one.  His eyes welled up with tears as he whispered "Did I flunk?"
I tried to explain looping, I tried to explain that there were both third and fourth graders in his class, I tired to point out that he would be with his best friend.  But then the step-touch started, the eye contact dropped.  I gave the teacher big eyes-like "what the heck?" but didn't want to talk about it in front of him.  I tried to send him in the room, but he refused to go without me.
There were no other kids in the room, plus it was his class last year.  This should have been a non-issue. 
He wouldn't leave my side.  So in we went, but he wouldn't look for his desk and started chewing on his hands.  OH NO!  He doesn't do asperger's at school anymore.  I tried to get him in the right mindset.  I encouraged him to go into his second grade class room and he see his best friend.  "I'm nervous."  (oh, I can't take it).  His teacher walked him next door and I broke down. 
Now, let's be fair.  I have had two awful days.  It probably wasn't just the school experience that had me in tears, but this crap is sneaky.  I had to back pedal so the teachers knew I wasn't questioning the placement, but that I'm sad because he "doesn't usually have" aspergers at school.  I'm a mess.  I should probably be committed.
I know he has the diagnosis.  I see how he acts in new settings.  Why does it shock me when asperger's shows up?  I don't know, but it gets me every time.
In the words of Forrest Gump, "That's all I have to say about that."
When we left the building, he had been happily playing with friends and was able to tell me two reasons he was happy he had been looped.