Who knows what the criteria are???
My exchange only serves 6702 residential and 278 non-residential premises yet is classed as market 3!
Judging by the dates which the LLU operators went live, your exchange will have been in one of the 'first batch' of Market 3's which were taken status at jul 2007.
Ofcom did a further review to update some exchanges.. and from what I can gather this may well have been when they started applying the 10,000 premises rule.
There may also be a few exchanges in the most recent list that arent yet M3, but they were projected to have been M3 by the end of last year. I wouldnt think you would come across many (if any) of those situations though.