In most cases, they were basically an "alternative" way to jackpot a clubber. Both being maxed doesn't necessarily mean they're ready to drop, since it's possible just to set them to max when you reset the machine, and technically still have to "build" to jackpot level (you just dont see it). However if they've built up over time, probably more likely to drop, but of course not guaranteed.
The reserve, presumably, is just to set the cashpot back to having something in it, so it's not at zero for the next player that comes along.