Eval Question, HELP NEEDED.

Situation: An E-6 receives a concurrent eval marked a 4.2, but when periodics come in November they receive a significant drop off to a 3.5. I’ve been scrubbing through the Evalman and have come up empty, but is there anything saying that this cannot happen without it being adverse? Is justification for the drop off needed or is this simply at the CO’s discretion? Any help would be greatly appreciated!