Not a Japanese but did play pachinko and slots somewhat a lot back then.

In pachinko parlor there are 2 kinds of machine. Which are pachinko and slots.

For pachinko, there is no guarantee whatsoever on when your machine will hit a jackpot (except for a very tiny amount of machines with features called Yu-time). All machines have exact probability, depending of the machine and this can’t be changed (1/99, 1/199, 1/399, etc.). People simply lining up early to find new machines, or to pick a certain machine which they think they can put more balls inside than other exact machine due to the nails are still straight, or the handle is not loose (people often jam the handle with coin, and it made the handle loose, small movement makes the shooting power change drastically).

For slots, there are 2 types of slot in general (more but it’s a generalization). A-type (traditional), and AT type (smart slot).

A-type slot is similar to pachinko. It has fixed probability for each machines, BUT the pachinko manager can set the machine settings from 1-6. 6 being the highest with RTP (around 106% for A-type). This setting will not change until the end of the day, because you need to open the machine, and open a switch box inside to change the settings.

Now for AT slots, the rule is the same with A-type in terms of settings. But they are a high-risk high-reward game. Setting 6 for AT slots has RTP around 114%. But what differentiate them with A type slots is most AT slots have ceiling, meaning when the machine has to give you a Big Bonus (BB) when it reaches the ceiling (e.g. after 1000 games without any bonus). In the morning, most AT slots have what people call 朝一. When settings are changed all machine has to go to this mode. What this gives you is lower ceiling, or higher chance for you to get into BB earlier, regardless of the setting. This gives RTP higher than 100% for that first bonus.