@Fantastiche Hure You have sent a big question that I have always done.
Personally I think in the Timeline "Alternative Present" Everything happened equal to "Present" we all know, except that the only time traveler was Trunks (The Future Trunks of the Timeline of Cell) . Then they fought against the Androids and discovered the device that stopped therefore destroyed. Later this Trunks return to your Timeline and with the same device just with their Androids. Well, in this "alternate present", after killing Androids, they spent 20 years and appeared Cell your Timeline and possibly this absorbed a lot of people, but not finding Androids, only reached the level it had before to absorb # 17, therefore I do not surprise me that after 20 years, Goku and Vegeta but never entered the Rosat, succeed in becoming more than 10 times stronger.