Well, it would be best to ask those who made the survey that produced these particular numbers.
In general, it would be my guess that it is because of lower motivation and lower efficiency of the public sector.
As for where that comes from - the government bureaucracy tends to pork out - become too large, cumbersome and inefficient over the years, in pretty much any country in the world.
Partially as a result of that, and partially because of certain other factors, people feel that the way they actually do their jobs doesn't make much of a difference in the governmental bureaucracy. Because of the "fat" system, individual work goals, as well as responsibilities, get lost in its complicated structure.
Also, if you're a private sector worker and don't perform too well, you will probably get fired rather expediently. The private employers are careful not to waste their money on paying laggards. The chances of this happening in the government sector are far lower, as the oversight is nowhere near as tight. Therefore, people feel they can afford laziness in government jobs. (And quite often, they can, to a degree.)
Hope this is of some use.
Misacek01 14:31, 8 June 2009 (UTC)