Magners,
I agree that the number of fires reported by the media do not add up but I can assure you that there have been more than one.
As has already been stated, galley smoke, burning oil and electrical burning will account for the vast majority of incidents, and prior to around 1995 there were also a number of retro fires.
I have personally experienced two real under-floors (one of which was on the ground), and know of at least three other fires that needed the use of internal fire extinguishers.