Volatility describes how a game spreads its payouts over time. Two slots with the same return can behave in opposite ways: one pays small amounts often, while the other pays almost nothing for a long stretch and then releases a large sum in one hit. This measure is rarely printed next to the return figure. Some providers rate it on a scale inside the help screen, and reviews often estimate it from a short test, which is too few spins to judge.
Many slots are released in several return versions, and the operator chooses which one to install. The difference between the highest and the lowest version can reach several percentage points on the same title, with identical graphics and rules. Nothing on the reels shows which version is running. The game's own help screen states the figure for the copy in front of the player, and it is the only reliable place to read it before staking.
Reputable providers publish the return range of each title and send their random number generators to an independent audit lab. The certificate names the game, the version tested and the date of the test. A licence usually obliges the operator to offer certified games only, which is a real constraint on what can be installed. The same licence rarely requires the figure to appear on the game screen, so the certificate and the help screen remain the places to confirm it.