One thing to sort out: WPS and Welder Qualifications have different essential variables as someone has already mentioned
There are two different tables, Table 4.5 for WPS essential variables, and Table 4.12 for Welder Qualification essential variables.
Essential variables are things that must be kept within these tables or requalification must take place.
It's easy to confuse these, but we have to keep them separate.
As they others have stated previously, if a welder qualifies with a single diameter, they are qualified to weld with any diameter-Table 4.12
Prequalified WPSs may have several diameters listed on a single WPS, but as you have stated, the parameters will be slightly different for each diameter. As Lawrence has pointed out, it simplifies things when you limit the use of a wide range of diameters in the shop down to one or two. I worked in a shop that welded a wide range of thickness and I limited it to two diameters. 1/16" and 3/32" FCAW wires...the 1/16" covered the thinner materials without burning through and the 3/32" was able to deposit a lot of material quickly to fill bevels for full penetration welds on 6" thick material.
FCAW and GMAW will have separate WPSs, each groove configuration should have different WPSs. You want your WPSs to be a useful tool for your welder, not just a piece of paper to pull out and dust off during a shop audit. I developed a welder's manual with all of the WPSs and fabrication notes required for them to do their job and feel comfortable while being quizzed by auditors. It really helped them find the joint in question quickly by placing the joint designation down by the page number so they could quickly thumb through and find the designation shown on the shop drawing in the weld symbol's tail. (ie. TC-U4b)
Educating the welders to read and understand these welding documents goes along ways to increased quality and production in a fab shop.