[Anthony]

In the end this is all about voltage. When fed nothing, the microphone will produce no voltage (or a steady DC voltage, but steady is the key word). Now when a 1kHz tone is played at 80dB a voltage is produced. When a 10kHz tone is played at 80dB a different voltage is produced. They should be the same, but most of the time they are not. This is why you have a microphone cal file.

So when a cal file is subtracted from a 0dB response, you get the result you are seeing.

Now you are saying, but hey, what about background noise in the room? That noise is most likely very broad-band and unless you have machinery that emits a high pitched whine all the time, the mic is likely just picking up a fairly even white noise.

If the soundcard is noisy (or the mic internals) the noise floor may be above the ambient noise in the room! Then nothing would get picked up.

Also, the soundcard cal file is only affecting the frequency response, not the noise floor.

Noise is a floor that you cannot measure below. It is random, usually flat, but not always. The important thing is that you cannot correct for it, you just make it as low as you can (insulation, better sound card, repeated measurements) and then make the signal much higher so the noise does not matter.

Frequency response is predictable and correctable. Different correction values are required for different frequencies, but because that is not random and repeatable, you can correct for it.

In this case, we're seeing no signal, so just noise, and the calibration file is being applied to the noise after the fact.

One last thing,
I have another thread about this microphone. While I did not test subs, I pretty much show that orientation does not matter for the mic for frequencies below 3k or so. Your measurements are probably good and I would not worry about it unless you want to start measuring full range speakers.