What range do you mean? Is there any condition that changes port supply to main supply?
On page 53, there is a table that shows port becoming less effeccient for supply due to range. In a way, I guess the easiest wait to put it is that you could consider a port at 21-30 range from units to be twice as small as a port at optimal range (0-10).
Each port will provide a certain amount of supply for units to consume it. If there is not enough for everybody's needs, excess units (chosen by the program) will be put on basic supply. Those units will recover very little readiness and can get no replacements or upgrade.
My guess is that last turn either you captured a port (or actualy, a port captured the previous turn became operational) allowing for extra supply to get in and removing the need for extra units to be in basic supply; or maybe you had just enough loses to reduce your supply needs. Units that have lost strenght (steps) will require/conssume less supply.