So for the last few years many (all?) orgs have combined rank groups. For example, instead of ranking 28/29's separately from 26/27 like we used to do - those groups are combined. There is a separate rank list for each CL but then HR merges them.
Normally you would think this would be a disaster for the (usually) younger 26/27 group as they would effectively be competing against more experienced employees for rank. But of course, the corruption is strong with XOM these days. HR "merges" those lists in a way that skews the 26/27 groups above the 28/29. The overall distribution is even but statistically the 28/29's rank lower.
It's a trick to give rank points to younger employees. Helps retain them I guess. But it also means the 28/29's get pushed further down and into PIPs more than the 26/27's. But hey it saves the company pension payouts so they are fine with it.