Because the mids and highs are enforced within the room lending a flat in-room response rather than a boosted response.
Now this of course depends where you sit in relation to your speakers. If you are sitting near-field, you are receiving mostly direct sound and hence an attenuated mid and high response. If you are sitting in the diffuse field, the response is flatter.
A-LFR/1000-8 & 2/500v4x2
Air Force, Freedom