I wonder if it's more common with science based PhDs for the Uni to claim the IP, as they are more likely to be able to capitalise on it than a social sciences/arts PhD? Hmmm....

I wondered that too. However at the first uni where I did sign away my IP I think this was a blanket rule across the board for all PhD-ers. My new uni seems to have the opposite blanket rule!

I'm in science and my supervisor's explanation was that many uni's will come to some arrangement (ie - share the spoils with the student) if there is a discovery that could lead to a patentable invention. If you haven't signed the rights away in your contract you could hold the IP, but what would you do with it? Certainly in terms of patenting the costs are way beyond the individual, a lot of universities don't even have the resources to patent their discoveries. In my field, unless you can sell your idea on to a substantial company, you'll probably get little out of your find. Both my supervisor's hold patents, but I know they've yet to make a bean from them directly.