Still confused by supply (after all these years)

[Pocus]

Why is the left HQ pushed to 8 by the right HQ? The right HQ, positioned at 8 from being adjacent to a harbor or located in a city, should provide 4 hexes away 4 supplies to the left HQ. Yet, this one is at 5 (pushed to 8). It is definitely not drawing supply from another source, from what I can see. Moreover, the right HQ is shown in yellow, as is the second supply of the left one.

Bonus question: What is the rule regarding supply preservation when running alongside a railroad? Does it decay?

[Lothos]

Why is the left HQ pushed to 8 by the right HQ? The right HQ, positioned at 8 from being adjacent to a harbor or located in a city, should provide 4 hexes away 4 supplies to the left HQ. Yet, this one is at 5 (pushed to 8). It is definitely not drawing supply from another source, from what I can see. Moreover, the right HQ is shown in yellow, as is the second supply of the left one.

Bonus question: What is the rule regarding supply preservation when running alongside a railroad? Does it decay?

Ok, I will explain

It does NOT matter how close you are to a port, what matters is how much RAW supply (not supply from another HQ) is getting to the HQ.

An HQ needs RAW supply 5 or higher so it can raise it to a supply level of 8

An HQ needs RAW supply of 6 or higher so it can raise it to a supply level of 10

In your screenshot the left HQ does not have enough RAW supply coming to it so it is linking itself to the nearest HQ (within range) which then allows it to get bump up to an 8

This feature is VERY helpful in areas like Russia where you are constantly moving the front and cities to provide enough supplies are not really available so linking is the only way you can keep pushing the front.