The changes in measured values near black tend to move by consistent amounts. Using xyY, for example, the x and y values define the color; and the color of black is not visible, so until the brightness (Cap Y) rises a bit, those numbers don’t have much meaning. The brightness, on the other hand, does what you describe, and tends to step by certain amounts: typically steps of about 0.013 between each step. This is as granular as Spyder X can measure at those low levels, so this is normal and expected behavior. Keep in mind how small those steps actually are, and how low the amounts of light are, which Spyder X is receiving. It is doing a much, much better job measuring these near-black levels than any previous Spyder.