Unlimited web hosting does have limits. The limits for unlimited web hosting accounts are hidden in the long unread Terms Of Service pages on the web hosting companies website.
If you read the Terms of Service you will see a limit to how much disk space and bandwidth as well as CPU usage that you're allowed to use for your unlimited web hosting account.
Most websites will never hit the limits of an unlimited web hosting account with their website as most blogs only use less than 1 GB of disk space and maybe 10 GB of bandwidth per month and that's what the web hosting companies are banking on.
They know that most websites will never need all that disk space and bandwidth or will never hit the limits so they use the unlimited web hosting term to get more web hosting customers.
But if you try to host a lot of websites on the unlimited web hosting account and start getting a lot of website traffic or you use more than 10 GB of disk space then you'll likely be asked to leave or have your web hosting account suspended.