I used the dice because it's the simplest form of gambling. Of course, machines are more complex, but the basics remain the same. They are not allowed to remember the previous "throw", but they are set on a certain chance. That means, if the chance to win is 98%, it is possible that a machine pays out less than that or more than that, because it's based on chance. The longer you watch it though, the closer it'll be to 98%. That's how propability works.
Also, I gather that those "fruity" machines only pay out what's in them? Because the machines I maintained payed out independent from the actual amount of money in them, as I payed the winners out from the cashier.
Now if they only pay out what's in them, it addas another "layer of luck", meaning that if you win on a machine that hasn't a lot in it, you've bad luck because you win only a little. That would also be the only thing that you can gain knowledge about by watching. Still, the chances to win should be the same, no matter how much money is in them.
On a closing note: gambling is a serious addiction that can and will ruin your life, much like alcohol or drugs.