did you try to take readings 1 meter to the left or right or front and back

my guess is you have some wicked reflection/phasing/cancelation going on. bass traps would help with 100hz but not as much on the 500 and 897hz areas.

try re-arranging your room better. what are the dimentions/materials of the room?

do you get the same spectragraph reading at lower volumes? my guess is no.