First, I apologise for the late response, and second for getting my maths completely backwards. I was tackling the inaccuracy of one half of your 'wasted' equation -and, in fact, completely the *wrong* half!
Had I not had an altercation with a piece of public transport, my next response was to investigate all the reasons why what Oracle reports as allocated to a table might not be found in the table's own blocks... but I see Jonathan has beaten me to the punch.
Thanks Jonathan: Iit would have taken me much longer to think of LOBs as a potential cause, anyway. (I was going to start by asking if the table was a cluster...)