Yes but alone an IP address gives you a pretty good idea, especially when combined with what sorts of stuff is being visited and behavior on this sites. There may be lots of public networks, but most people still browse the internet at home, with a small number of people on a single IP.
Additionally, as above, these different devices can usually be distinguished from each other by comparing the browser request headers. This gives an additional heuristic to determine who is doing what.
This is a situation where you have a bunch of hints, and any one may not get you that far. However, if you combine the information from all of the available hints, you get a pretty clear picture. The military likes to call this type of situation 'total information awareness.'