With the current amount of information in your question it's going to be hard to state anything with certainty. But I think your issue may come in a difference between your understanding of the statement 'potential research projects for prospective postgraduate students' and the professor's intention behind that statement.
This is given with the caveat that I can only speak for my experiences and the experiences of those I know in similar fields. Nothing said here should be construed as fact or as the only option.
Many professors have a lot of research interests. Typically when you read something like 'potential research projects for prospective postgraduate students' that means that this is a research field the professor is interested in pursuing. Obvious right? What's not obvious is that this does not mean that the professor/lab/researcher has begun research in this area.
To take a directly related personal anecdote (with some details changed to protect the wicked)...
One of the professors in a lab I know has done a lot of research in the field of markov random fields and motion planning. This includes medical research using markov random fields, computer vision work using random forests, motion planning comparisons... etc. Hopefully you get the point; this professor has tons of work in the field of Artificial Intelligence with a pretty heavy bent towards robotics.
This professor also has a grant to explore human robotic interaction specifically with regards to direct or hybrid human control over robots. This is not directly related to any of the professor's previous work(though other work may have brushed up against this area). Additionally none of the professor's current students are interested in this area of research(already being involved in their own projects in other research areas.) The professor is now looking for students to do research in this area.
In this way you can have a professor/PI/researcher/lab that is looking for a researcher for a project that that particular individual or group has not yet published in.
So to be a bit more specific about your question. Is this particular research area a deal breaker for you? What I mean is - all other things being awesome, if you cannot do this research in this lab would you turn down a spot in the program? The reason this is important is that it could change how you interact with this professor.
I think the thing you should be most aware of coming into this situation is that sometimes research doesn't work out. This is especially true when a lab is entering completely new territory(an area of research it has never been involved with before). If this particular area of research is a dealbreaker for you(it is the only research in that particular lab that you are willing to work on) then you should be cautious jumping in. If somethings comes up and that research is, for whatever reason, not continued you need to ensure that you have other options or you will find it very difficult to finish your postgraduate degree.
Finally, it is not rude or inappropriate to contact a professor asking for more information about one of their projects or interests. What is rude is not doing your homework before hand. It's the difference between "I think cats are neat, tell me all about cats" and "I've done reading into cat psychology and I think your project on why cats put their butts in your face is very interesting. I've read but I was hoping to get more information from you about it." Professors(most professionals and experts) are busy people and no one wants to feel like you haven't done your homework. If you approach such communication with professionalism and courtesy then asking questions is almost never a problem.