RP's race numbering system - anyone figured out the logic behind it?

Hi,

If you look at today's URLs for UK races they have race number of 529xxx as in:

http://www.racingpost.com/horses2/cards/card.sd?race_id=529802&r_date=2011-05-14

but there are races - which are a couple of months old -  with numbers of 533xxx.  If you've pulled any data from RP you'll know it's not uncommon to have large chunks of sequential numbers not allocated to any races at all!

Anyone figured out the logic behind the numbering system?

thanks