The box speed is great for people who use it as-is... and people who push their film to shoot in dingy dives can double or quadruple it if they like...
Box speed is also useful for many, many purposes, because it is founded on results.
But Zone System is a system. A System intended to get a certain kind of negative for printmaking using certain metering techniques.
If you're not going to use Zone System, you can use ASA/ISO speed as-is and get good results most of the time. But soon as you change some part of your metering technique away from the average, you may need to recalibrate.
Say you do something as simple as pointing your camera down to avoid getting too much sky in your meter readings. Now you messed with the average and a "different" speed would apply to your practice.
So it's OK for Zone System speeds to be lower, while the rated speed stays what it is.