Still there some people who are offering unlimited diskspace, bandwidth ? When it's impossible why do they claim this?
Probably because they are new to the industry and havn't done their research. Or maybe because they still think it's a valid marketing technique. There could be any number of reasons why.
The average website doesn't really get very many visitors and doesn't take up much space. Most of my websites are good with about 5-20 megs of space and less than 1Gig of transfer.
They say it because not only is it a marketing ploy but it works to get people to signup. Even though people are becoming more and more educated about these types of issues, if hosts are still offering it it's because it is still working. I'm not sure there will ever be a day when nobody will claim to offer such a thing. Of course you won't see those offers here -- or at least not for long.