Interesting. Wonder if this is true with the machines that you drop quarters (or tokens) into trying to knock a bunch over the ledge?
The ones at carnivals are the worst (they usually pay out in cash). If you look closely at the walls on the sides of the table the coins are on, you'll notice that they are open just enough for the bottom layer to fit through. The physics of the game dictate that the more coins you have in front of the new coin being introduced, the more forward resistance there is, which causes the coins to be more likely to squeeze out the sides. The con artists who run these games understand this perfectly, which is why they dump a fat pile of coins, usually with a $20 bill in the mix to entice, right at the front edge of the table. This adds more weight to the front which causes even more coins to squeeze out the sides.
The sad part is that there is nothing really hidden, except the visual of the coins falling off the side (but you can hear them). Yet people just keep dumping their money in.