The "mean" value referred to in table 3.1 is a value the WPS writer needs to come up with, as long as it meets the other requirements for a prequalified WPS.  All the table is trying to say is that you cannot write a WPS with excessive parameter ranges.  Such as if you were writing a FCAW WPS for 1/16" wire, you might have an average, or "mean" amperage of 250A, so with one WPS, the amperage range could be 225-275A.  If you need a lower amperage procedure for out of position welding, or thinner material, and want to use 180A, you would have to write a different prequalified WPS.