i believe it's the average reserves of that world.
if the average amount of oil people have at the moment is 5k then the reserves will be 5k
so at the start of the ticks not many people have much, so it's at its lowest which is 3k
right after boosts, people just received large amounts of resources so the average is higher and the reserves are usually at their max 6k.
Eh, I try to win every once and awhile. Gotta keep up appearances.