the weirdest thing I've ever come across

Hey Guys,
I have a mu set up and till date I've had no problem with mapping domains within my set up as majority of them are bought with the same hosting provider,so i just pointed the ip to my dedicated ip .
Lately a client of mine has provided me his domain which was bought else where and I've had pointed the @ to my dedicated ip and till date its never appeared to me that whenever the site is typed like mysite.com it shows up but when I type http://www.mysite.com it takes me to the domain providers site ( I have parked it as normal by pointing to my ip )
It only came to my attention when I was trying to verify the site with google !
Can someone explain me this mystery ??