Seem to get worse DPS results without manual forcing of items (TBC)

i have no clue why but the TBC calculator seems to hate putting meta gems into gear.
it uses a cowl of the grand engineer for its BIS, but it yeilds nearly a 1.5% better result if I force a helmet with open meta socket.
is this a flaw in the bug? since 1.5% better DPS if I keep my current helmet with meta over a new helmet is a weird downgrade.