The manufactures claim are estimates at best. FALSE !
All the math modeling and flow test are done at all the manufactures, more than one way at that.
I guess they should throw away the flow test machines then. Their tested the same way as a head for flow.
I wish I could guess that close.
I made flow tester's for the oval one on the V8 back in the 80s and could measure the size of a BB difference between ports.
Even Flow must be controlled for all the cylinder's, this is the job of the intake.