Yes. I haven't for years looked at departmental estimates in the raw to know what happens. My suspicion is that pretty well every department has a different way of doing it, and there's some argument for that, because the things they're looking at are different.
But some understanding of the methodology behind cost estimates, which is something I've been impressed by with the current Parliamentary Budget Officer, is a great help in understanding what's being projected and in having confidence or a lack of confidence in the results.