Disparity in official figures for civilian killed by drones

How many civilians in Pakistan have been killed in drone strikes? Leaving aside the question of who is a civilian and who is a combatant, there is an enormous disparity, as pointed out in an article by Justin Elliot of ProPublica - not least, in those given in off-the-record briefings by Obama Adminstration security officials.
Bill Roggio, editor of the Long War Journal website, which bases its estimates on news reports, puts the number of civilian killed in Pakistan at 138, against 2,307 Islamist fighters. The New America Foundation states that, based on press reports, between 293 and 471 civilians have been killed in the 300-plus attacks since 2004. 
The London-based Bureau of Investigative Journalism, which draws on sources including researchers and lawyers in Pakistan, puts the number of civilians killed at between 482 and 832. The authors of the various estimates all emphasize that their counts are imperfect.
Elliot has used another indicator to analyse civilian casualties. He has compared the many claims made by Obama Administration officials about the number of civilian casualties. His research shows that a wide variety of figures have been given out - off the record, of course - over the last few years. These include "a handful", "zero", 60, 50, "about 30", "under 40", "at most a few dozen", "roughly 30" and so on.
US security officials - the CIA - claim they know the figures because they monitor phone and other conversations after a strike and they observe funerals and other events connected to the strikes. Sounds good, but with such a variation in figures, their case is undermined.
For some people even a single civilian death is one too many whilst for others the issue is that of proportionality - whether or not the number of civilians being killed is proportional to the military advantage anticipated. For those running the CIA drone campaign in Pakistan it seems that the only figures that matter are the number of senior al-Qaeda and other militants who are being killed.