TheMadScientist - 3:34 pm on Mar 17, 2010 (gmt 0)

What could be a reason for that?

Data drop in the data pushing process.

"Google does not want to tell"?

Could be.

Some types of pages might be dropped out of the published data to discourage link buying / selling purposely. Could be a connection thing where the number is cached by the requesting resource for N days (months) and was not present on the initial request and has not been re-requested yet.

Could be a the page(s) got detached during the calculation estimating process and did not get a published score this round.