Why is Syd called Syd and not Roger? I mean, yeah there can be two Rogers in the same band...I always figured it was his love of acid. Maybe it was an old nickname that stuck?
I think there was a guy in London or Cambridge back in the day named Syd Barret whom he particularly admired and so people started calling him Syd and it just stuck.
Bong is right. People had actually started calling him Sid Barrett, and he changed the spelling to differentiate himself, to Syd. Nowadays, though, he's just Roger.