"Why were these amazing employees not shown their worth in focals? did they have bad managers, were they too busy doing actual work, were they not the popular kids...?"
The irony is that I'd bet many of the people ISP'd had excellent descriptions of their accomplishments in their FOCALs. Unfortunately, the FOCAL write-up's weren't part of the criteria. Only the grades were.
There are many reasons that someone could be given Successful/SL4.
In my case, my manager gave me this because she said that I'm new and SL4 is the appropriate stock level for new people. I tried to get it changed, but she wouldn't budge. She gave me this despite some amazing accomplishments (incl. external awards and accolades in my Focal write-up).
At least in my group, the people who have been designated the "chosen ones" will get the higher grades as long as they stay part of the "in crowd" with the manager and her boss. Everyone else is relegated to scraps, no matter what they do or document in their Focals.