I wondered about this too and was thinking the micrometer adjustment may have a range slightly greater than the 1/2" increments, say +/- .0325 or .125". So that if your set at 1/2" with the micrometer you can adjust from .4375" to 1.125" or on the micrometer from -.125 to +.625 without having to snap to the next 1/2" increment.
But that's pure guess, my gauge is still in the mail and won't arrive for a couple more days. After seeing this thread when it got resurrected I watched the YT video and ordered one direct from JessEm. Their price was lower than any other place I found online, some sellers were 50% (+ shipping) higher than JessEm from whom shipping was free.