Quick calculation gives that you most likely still have the default 0.3mm bottom layer this gives 1 (0.3mm) + 20 ((1.5 - 0.3) / 0.06mm) = 21 layers as you saw.
Note setting first layer height to 0.06m also requires a perfect calibration of the print bed for the first layer to stick well, with a first layer of 0.3mm this is much less a problem.
Check your first layer height on the second tab.
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