I think I seen where the author was interviewed about what made them unique, and said something a long the lines of "ferb has the obvious signs of an autistic spectrum which obviously means since he has spectrum, he has the ability to listen to others around him instead of fully conserving the whole illness itself". Basically he has no motivation to socialize because, he simply has nothing to say." phineas has Asperger's.
To put his interview shortly he said it was to show there was nothing actually wrong with them like people thought, but despite the fact they both had different problems, but were not "Down syndrome" like most people assume. Edit
I read this in a blog once though, I'm not sure if it's true or not. So, don't count it as fact. But, it seems reasonable cause you can see it.
((Just because Ferb doesn't talk much doesn't mean he has autism, he could just be quiet. However there is more proof that backs up this theory. I am a proud father to three sons: Ian (6), Conner (5) and Jackson (2) Ian and Conner happens to have Autism and I can clearly see some of their symptoms in some of Ferb's behaviour. I would say Ferb has more mild autism like my son Conner, because when I was watching the show he reminded me of him. Ian is more on the severe end of the spectrum and Jackson is not showing any symptoms of a autistic child.))
((Hoi! I edited this!! I'm autistic too, and I find this very possible. Boi!))