I thought about deleted posts, but unless there is a plague of spam which the mods are dealing with without me noticing, the discrepancy seems a bit large.
Just had an idea: when the Custom Software forum was split off, a load of cross-links were established which then expired. Could be the difference?
Presumably the post count covers all the sub-forums, so somebody could claim the 10kth without even knowing they are in the game!
There were 136 to go when the lastest post link said 10085 - so pending correction as it gets closer (and we'll have to monitor to see if it diverges) the target post link is 10221.