I once observed the top candidate off a promotion board get there on the merits of "B+ Yes" with an honest and praiseworthy narrative, replete with highly compelling evidence. The smattering of A/A- Excp were often scoffed at under mutterings of over-marking, especially when evidence of Perf/Potential lacked.
The promotion system is very fair and those who observe one are almost unanimously in agreement of that statement.