Copied from the Nasa.gov site 'Where is Webb?'
NOTE: Segment A3 and A6 will be moved separately at the end of the process because their position sensors are read out in a different way.
Why would they be different?
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See 2018 SPIE paper by Wolf et al. "JWST mirror and actuator performance at cryo-vacuum" (find it by searching on "JWST A3 and A6 position sensors").
See sec 1.2 and 2.4.2 of the paper.
It turns out that the mirror segments at A3 and A6 have "faulty" linear variable differential transformers (LVDT) (position sensors). Each LVDT is suppose to have two coils to cancel out differential thermal effects. In each of A3 and A6, one of the two coils is faulty. The engineers had to work out a way of getting accurate readouts using only one coil in each LVDT. So they developed a different readout procedure that could use only the one good coil in each LVDT. I do not know why it was impossible to replace the faulty LVDTs before launch.
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6$\begingroup$ I wouldn't be surprised if they didn't know that the launch wouldn't take place until 2021 in 2018. At the time, they might have thought that replacement would lead to launch delays :) $\endgroup$– AJNCommented Jan 16, 2022 at 2:10