No, they can’t just change them on a whim. It requires opening up the machine and replacing a chip. Slot machines have different chips in them, depending on the payout percentage.

If you were wondering about online slot payout percentage, this site may be of use:

Only when no-one is looking.

Can they? Yes. Do they? No. The state gaming commission would have their heads if they did that.

Most slot machines these days come with a choice of pay percentages in them. These depend on the jurisdiction they are in. Access to this is also restricted depending on jurisdiction. All jurisdictions have high and low limits as to what the machines are allowed to pay back.

Nevada has the easiest rules for changing pays on machines but probably don’t since it’s in their best interests to provide consistency to their patrons. They don’t need to change them since they need winners. They make their money anyway.

Probability just works out that sometimes the machines just pay better.

They can change the percentages, but it would require them to physically open up the machine and tweak the program.