There is more than one way to measure the length of a rifle barrel. The nominal length is just a reference for making an order. Some smiths may not even ask length to cut barrel to, but length to cut off a barrel.
The NIST standard is closed bolt face to crown, or a measurement taken parallel to axis of the bore. I don't know that every manufacturer follows this, especially if they are not even a US company.
If Tikka sells me a 24" barrel, I believe they measure from tenon to crown. There is about a 1" threaded tenon, and 23" of barrel visible from action face to crown. If I want 20" of barrel to the action face, I could ask the smith to cut off 3". The overall length of the barrel per manufacturers reference would then be 21", after cutting. I doubt the bolt face to crown measurement also comes out at exactly 21". I would expect it to be tens of thousands of an inch beyond exactly 21".