The edge the bookmaker would have IF the odds guaranteed him a constant commission regardless of the outcome.

The theoretical hold percentage, colloquially called “theo” (synonymous with PAR, as well as theoretical win percentage), is the machine’s expected hold percentage, based on its internal configuration. However, there is no guarantee that actual game results will match the theoretical configuration, just as there is no guarantee that if you flip a coin 10 times, you’ll get exactly 5 heads. The more individual trials, the more likely that collective game results will eventually reflect the theo, but an individual machine can diverge from theo for reasonably long stretches.