Could be a lot of reasons.... at my last workplace:
porn sites (as in actual actresses) weren't blocked
movie rental places weren't
pay for sex sites weren't
news agencies were
search engines were
Until they fixed it! Funny... the blocked sites too a few hours for anyone to notice, and I was the one who complained... and there werew over 100 people using the internet that day too...
(Before you people start to think of something... I was in charge of the proxy log [internet history for all users] so I know what people went to!)
It could just be a weird glitch, ip address range block, server (hosting company) name block...