It probably depends how the regulations are written. Can the odds be _better_ than advertised without the casino getting in trouble? Then they'd quite plausibly have a range they could play with.
Slot machines, however, are opaque because the probability of hitting any specific combination is unknown to the public. The machine can promise that a 7-7-7 result will pay $5000 but you have no idea how often a 7-7-7 is supposed to hit.
Sometimes a casino will take some less popular machines and configure them with an extremely low payout percentage (3% for example) and because the players would not otherwise know this, the casino will put a sign up "97% payout!" to try to attract some value-oriented players, just like a retail store takes old merchandise and puts it in a bargain bin at 50% off.