I remember reading some disclaimer (presumably unofficial) when Avantix 95 was found that stated all fares in the system for £999 are fictitious and do not exist. I assume that, as Mojo as already said, these fares are for the operation of the system.
Saying that, though, I'm sure there are cases where if you incorrectly select an adult or a child for a ticket it comes up 'No fares for these parameters' or however it's worded as opposed to the £999 'fare'.