I assume you're thinking each compound has a threshold dose under which it does nothing. But if you instead think all these steroids work similarly then each compound adds to the total dose = more results.
In the case of tren, I knew plently of guys back in the day who did one amp of Parabolan a week and claimed to get great results. In the case of adding it to 150mg of test, it could make a big difference, it being so potent per milligram, much bigger difference compared to adding it to a gram. Adding 75mg of tren to 150mg of test might be the equivalent of tripling your test dosage.
I'll give another crazy hypothetical example of overlapping effects of different roids. Say you did 50mg of 60 different steroids totaling 3 grams per week. Each by itself too little to do much. Would the 3 gram stack not work? I bet it would work like a high dose cycle. Would be an interesting experiment to try, just mix 60 different roids in a 1000ml bottle, do a few grams a week and see what happens lol.