the air through meter design
Ohh I think I get it now... I just had one of those 'aha' moments
The meter tube has smaller volume than the volume of the chamber so you don't need to set the reg to 6.44psi but instead you can set it to ~64.5 psi (provided that the meter V is 1/10 of the chamber volume).
'Air through' means that air flows to the chamber via the meter - thus mixing it.