In most electromechanical pinballs the match unit cycles downwards when the tens score unit moves upwards during gameplay (earlier EMs its linked to the ones score reel not tens, and in some both move upwards{this changes my next statement below}).
Interestingly, if you look at the numbers, it basically means you have a 50% chance of playing a game where you have a 2/10 chance in getting a match or a 50% chance of playing a game where you have 0/10 chance of winning a match - with the idea that it should average out to 10%. By looking at what the match was for the previous game you can figure out your chances of matching for the next single player game.
Some machines such as a lot of Williams EM's didn't set the match unit to be sequential, they followed a pattern 60 10 70 30 80 20 50 90 40 00.

Early solid state machines were very basic, they basically were just a random number, or some digitally replicated the above EM system.

"High Speed" (1986) introduced the options for an operator to adjust the match win percentage. Your chances of winning are affected by who is playing and when. There are absolutely heaps of variables that can affect the match chances... over time these algorithms have got more and more complex, but basically the machine will try to keep the match win percentage within a threshold, if too many games are being won then your chances are lowered, same with if not enough games are being won then your chances are increased... so its still a random number, but imagine its like picking a random number between 0-10 then if too many games are won you start picking a random number between 0-25, so there is now less chances of a match......  there are some interesting scenarios that can affect the chances of matching such as:  there is an extremely low chance to match if you win a free game by a high score... the match chances can increase the longer a machine is idle to try and grab more attention when someone finally plays it... scenarios such as if there are 2 players and 0 credits left then there is a much higher chance of one player matching to try to get the other player to drop another coin in.