I think it has to do with more work being "creative", in the Richard Florida sense, of being problem-solving oriented. It is much easier to estimate the time needing to complete tasks such as "milk the cows" or "build a shed", because 1) You do them repeatedly and can learn, and 2) The bottleneck is not thinking, but the manual labor, or the time it takes to use a given machine.

I might be irrational, but I always estimate problem solving time to be much smaller than what will actually be needed. Even afterwards, I will usually consider much of the problem solving time as time wasted, because I should have been able to do the task much faster.