"Unlimited" means nothing more than "we're not telling you what the limit is."
It's marketing. If you use more memory, processor power, bandwidth, whatever unlimited feature; you'll be cut off and told to buy a bigger plan. The limit is usually higher than any shared-host customer is likely to need, but it's still there. This is always the case, regardless of which host or plan we're talking about.
But anyway... 🙂
Also, being on a shared host, it's entirely possible someone else filled up all that unlimited database space before you could.