What it comes down to
With prototypes and custom work there is no history and often only one piece. The machinist's call is then the yardstick. In Blake the time per operation sits on its own line, with time, rate and quantity, so the machinist can hold his own estimate against it and see line by line where the difference is.
A prototype shop has little use for features it never touches: a solid base that does what it should is enough. A correction with its reason stays on record for the next similar part. More on CNC milling and on estimating.