My guess, the Free Cache powerup is the base for all percent-based supplies artifacts. The supplies obtained from the Free Cache powerup vary depending how far you advanced into the level - early in the game you get few supplies, later you get way more. The amount is probably your money/armies/resources income for a specific amount of time, but I haven't studied it yet.
As i've noticed, armies given from Free Cache are strongly corelated with current army camp production. You can see signifacant difference when using FC while Supercharge Army Camp is in effect (especially during early stage of game as with very few army camps boosting one of them make bigger difference)