mydomain.com vs www.mydomain.com?

I just published my site under my existing domain name, i.e., www.mydomain.com. I changed my dns server to point to 208.75.208.172 as directed. It worked right away, however, it published under mydomain.com instead of www.mydomain.com. When i type in the www.mydomain.com, i still get forwarded to my old site (i had used forwarding/masking service previously). But when i type in just mydomain.com i get to my new synth site. Why is that? BTW, I am delighted with what i have found here at Synthasite, keep up the great work!