Rng is random number generator. Basically, it's just a way of saying some things are random.

On the topic of subs, I don't train my main team to get the bonuses, only my subs. I train them all at once, using drills to get all four of the bonuses up. After that, I see how much time until the next game, and then take time to train my main squad, so that I still have 99% when the match comes around, and have the full 40% bonus.