In the case of most progressives, there is just a single bank of machines, and they are communicating in realtime (wire delay) between them. When I quit the industry, the Megapot was the first server distributing money across sites in realtime (network delay). There was a similar product offered by IGT at the time, but the incrementing counters were entirely based on play within the casino; casino jackpot sums were batched and shared across the casinos according to a fixed schedule. Our displays were certainly based on real money, and I suspect anyone else in the industry would have to do the same to be legally in the clear.