"Fremeau Efficiency Index ratings (FEI) are opponent-adjusted possession efficiency data representing the per-possession scoring advantage a team would be expected to have on a neutral field against an average opponent."
Keyword being 'expected'.
Meanwhile ALL the other associated ratings listed there they grade well in:
DDE: ranked 29th
DTE: ranked 39th
DVD: ranked 28th
DFD: ranked 30th
DBD: ranked 22nd
DTO: ranked 10th