Slots are set with odds based on % of money going into them. They could have a hundred Wheel of Fortune machines, and all would pay out at different rates depending on $$ played in them.
The slot machines are set by a computer chip to payout at a given percentage. You are correct there. However, each pull of the lever/push of the button is a separate event. There is no memory of prior play and it does not matter how much money has been played previously. If the chip has been set for a 98% payback then the odds for every pull are 48/52 in the house's favor.