I think, reading some local press reports and Facebook posts (not good, I know, but all I can find) that Kingston Hill was eventually left out of the "blanket" 2023 imposition of 20mph limits. Hence the later order.
I think it's just a question of where, exactly on Kingston Hill that order specified the 20mph limit to begin. I cannot tell from GSV and your photos whether the sign is located "150 metres east of its junction with Warren Road" or not - particularly as that spot appears to be in somebody's back garden.