Fork oil level only acts as a kind of spring assister as the forks compress, giving the forks a progressive action, even with straight rate springs.
This is further complicated by some (progressive) spring suppliers wanting the tighter wound part of the spring down, othere wanting the tighter wound part upwards - this will have an effect on oil level with the spring installed.
It could be one manufacturer has a softer spring & the extra oil height helps bottoming, it could be their testers deem a firmer ride.
Who knows which is best, it's pot luck without extensive side by side testing.
As above fork oil viscosity is another variable to muddy things, best choosing by cst