At the time, it was explained to me that under a certain income level the school uses an alternate formula, that I presumed did not look at the same variables. I am not sure if that school still does this, or if they have since tweaked their other formulae as well. But this was not the only school where we found this anomaly.
I recall asking because at the time, DH’s employer would have allowed him to take a year end bonus on 12/30, or a slightly higher bonus on 4/1. Deferring it was better for our FA situation, but we had medical expenses so we had to take it earlier.