When do a particular progressive slot machines "must hit by" meters increment?

This is my first post in any forum here, so hopefully I've posted this question in the right place.

I have a very specific question about a very specific slot machine game. It's IGT's 3-reel Triple Double Diamond Progressive Free Games, which runs on their S3000 cabinet.

Background: On this machine, besides the base games, you can win one of three different color (red/green/blue) bonuses by getting the bonus symbol on all three reels. Getting all three reels with red bonus symbols means the winnings in each game in the bonus round are multiplied by 2; for green it's 5, and for blue it's 10. There are meters, one for each color bonus, that display the number of free games you would get if you win that color bonus. The number of games for the red bonus starts at 5, and over some period of play, increments one game at a time as high as 15 (if a bonus colors meter reaches 15, the bonus automatically hits). For the green bonus, it goes from 7 to 15, and for the blue from 10 to 15. Since a green bonus pays significantly more than red, and blue significantly more than green, most bonus rounds are red, less are green, and the least are blue. Similarly, the frequency that the meter for the red bonus is incremented (by one each time) is higher than for green, which is higher than for blue. So when playing the machine, you can observe the number of potential red bonus games being increased once in a while, the number of green being increased less often, and the number of blue being increased least often.

I have viewed multiple videos of games played on this machine in order to try to figure out on what basis the meters are incremented. It doesn't seem to be strictly time-based, or amount-of-money-played basis, or certain-symbols-appearing-on-the-reels basis. Certainly there is probably some randomness built into the decision on when to increment each bonus type, but at any rate my question is: Does anyone know the "rule(s)" for when each bonus types number of potential winning games is incremented?

I hope I've explained this well enough. Thanks for any insights.

Quote: Rav

It doesn't seem(s) to be strictly time-based, or amount-of-money-played basis, or certain-symbols-appearing-on-the-reels basis.

pretty sure its a part of every bet made

Please delete.

Some must-hits are incremented on coin out instead of coin in. It is much harder to time them.