Frequency Sweep Analysis Issue/Lowend Lower in dB

So I create a frequency sweep in Adobe Audition 3.0.

5-22kHz for testing gear..

Whenever I do an offline frequency analysis of the sweep, how come the lowend up to 160Hz

is rolled off very slightly...

Even when using a spectrum analysis plugin , set to averaging, to maintain the trace.

Then loop and play the sweep, to create the trace. I still see the slight rolloff in the lowend.

Since all frequencies are supposed to be equal volume in the sweep, why are some showing up at a lower level in the lowend?

Changes depending on if I select Log or Linear sweep...