I think it's entirely based on the basic stats. Not as a rule or anything, just as a holdover.
The arbitrators aren't necessarily baseball people, they are labour law people (lawyers and judges), so the process is kind of stuck with with these old stats and old reference points that can work across both the baseball and employment law worlds. That's why they also care about playing time and awards.
"I see here, Mr. Donaldson, that you barely missed any days of work in 2015 and you were also recognized by your peers as being excellent at your job. Very good!"
I don't really know how it works exactly.