It is possible (but highly uncommon) that an internal page has much more links to it than other pages (fantastic review of something/really good link bait) added with low bounce rate, etc. resulting in a higher
PR than the home page, BUT your internal site structure must be way off. Did you inherit/buy the site? it seems like this would be a feasable explanation. I would review the whole internal link structure to ensure a clean and proper structure (try not to have more than 3 clicks away from the home page as a basis and work from there - I have no idea as to the size of your site). The site is obviously doing something right, and if you're PR5 and are having to ask the question, I'm guessing it's not yours or you recently acquired it?