I'm not sure whether it's something that happened this year, this year the Bryans are ahead of Bjorkman/Mirnyi in the race.
Overall, they're pretty close.
Full breakdowns for Mirnyi and Bob Bryan are here:
http://www.atptennis.com/5/en/players/playerprofiles/doublespointsbreakdown.asp?player=M595
http://www.atptennis.com/5/en/players/playerprofiles/doublespointsbreakdown.asp?player=B588
The ones for Bjorkman and Mike are slightly different, since there a couple of tournaments of discrepancy, but overall they're pretty similar to their partners.
Mirnyi has a total of 6205 points, of which
1000 come from winning Roland Garros
750 come from winning the Masters Cup
700 from the US Open and Australian Open finals each
A couple of 500-point wins at TMSs
A bunch of 155-225-250 results, like the quarters of Wimby and semis and finals of TMSs and a minor-tourney win or two.
Bob Bryan has 5675 points, of which
1000 for each of Wimbledon and the Australian Open (wins)
700 for an RG final
Two 500s for wins at TMSs
A nice progression of results from 125 to 350 points for things from USOpen R16, a couple of minor-tourney wins, some TMS semis and finals.
Pretty close, overall. I can't pick out one thing that stands out to make one better than the other.
I guess Mirnyi has four 700-points-and-over tournaments on record - a GS win, two GS finals, and the TMS win. Bryan has three - two GS wins and a GS final. Overall, it seems Mirnyi's results don't taper off in point value quite as fast - he has only one tournament being counted worth 155, and the rest are 225 and over, whereas Bryan has the lowest 5 of his best-18 being 125, 150, 175, 175, 200.
Not sure what to make of that - it's pretty close, it's just coming down to point values and technicalities.