Ah! Ok so just for supply level, not taking into account HQ rating and pip.
Indeed, it shows correctly if the supply could be better by attaching to another HQ, but the detachement to the "wrong" HQ doesn't reduce the supply lvl itself.

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3 of those fighters are detached to the HQ north of Tours, which provides 8 supply. The HQ in Tours would provide 10 supply and be the better choice.

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This fighter receives 6 supply, it is detached to the HQ in Orleans. Orleans is providing 5 supply (1 less than Tours, diminishing the supply provided by the HQ by 2). This also shows that a) the display is correct and not related to the sheer distance; another HQ further away from the fighter would indeed provide better supply, and b) the supply isn`t reduced only because of the existence of better choices; it's as expected (provided supply minus movement costs).
The whole situation shows that 1. the displayed purple colour is indeed a helpfull tool (sadly it shows the development when it's mostly too late) and 2. the reason why I hate the change of the original supply mechanics. It's so inconvenient and requires such carefull planning to get the best result, I find it really frustrating and also completely unnecessary, since it's only reason is to articially reduce the power of aircraft. While that might be kind of reasonable or at least debatable, there are more convenient ways to reach that goal imho.