An estimate is really a probability distribution, that often resembles a Gaussian distribution and thus can be captured with two numbers: the average and the variance. Combining multiple estimates together would involve some math around these two numbers. But the surprising thing is that managers in the software industry don't really understand probabilities, and to make it understandable those managers want to reduce the model to just one number, and when this model fails to capture the probabilistic nature of the underlying process, they attempt to overrule the math with authority.