Jacques, to the extent that your meter is accurate, "Flat" will be correct at low frequencies with regard to what the SPL level actually is. Since human hearing becomes progressively less sensitive at low frequencies(except at very high sound levels)the "C" weighting at typical levels more accurately duplicates the ear's sensitivity. For example, the C weighting is 3dB down at 31.5Hz, and a reading there using that weighting should be(intentionally, not an "error")3dB lower than a flat reading. "Correcting" this by adding 3dB isn't correcting an inaccuracy, but simply offsetting the effect of the built-in weighting, leading to a subjectively more nearly equal loudness.
These factors vary with frequency and sound level at each instant and a major benefit of Audyssey Dynamic EQ is to provide more sophisticated adjustments at each moment, with resulting significantly improved subjective bass response at very low frequencies.
