I hate it greatly when people won't say "I don't know", but instead hem and haw and avoid the question or give obtuse answers. Once someone admits they don't know, it's fine, you can work to solve the problem another way. But for that to work with most people, I think you have to be pretty nice and make it clear you are not the sort of person that is going to cause problems if they are honest about ignorance.
There could be because they got punished or saw someone get punished for saying "I don't know". Or, they either got rewarded or saw someone get rewarded for fudging and lying their way through when the real answer was "I don't know".
It is also about trust. When there is trust between the employees, one is not afraid of being honest and saying "I don't know" followed by "but I will find out" or "ask Jack, he knows more about this".
If there is a feeling that punishment or implied accusations of incompetence or experience will be made, they will start making shit up on the spot.
I see this everyday. It is really sad, hours of productivity is lost because people can't be honest. Usually it goes like this. A new employee, call him N, doesn't know how something works. They don't even know who is responsible for that product area. They go to a random employee X to ask for help, so they ask "how does this work?". Employee X knows they themselves don't know, but they won't admit it and won't send employee N to the employee Y, who really knows about the stuff.
Instead employee X starts to make stuff up on the spot, or which is slightly better, they both spend hours learning about the answer by reading code (not too bad again), but all they could have done is asked employee Y about it and they would have gotten an answer in under a minute.
That should be easy to fix, all you have to do is to ask them who knows the most about the question you have - then they can help you without being dishonest or losing face.
I do this as a job interviewer sometimes - keep asking questions until the candidate says they don't know. The ones that behave as you state in the first sentence become no-hires.
