Split time lines. T1: Goku dies, the Androids kills 98% of Earth's population for lulz. Trunks goes back in time, creating T2. T2: Goku gets medicine, survives, they train, the Androids appears, they are defeated, they find Geros HQ, they find the turn off switch, Trunks brings it to the Future, turns off the androids, destroys the remains. Cell wakes up, sees that the androids are dead, kills Trunks when he is on his way back to the past to tell his past friends that things are alright, goes back in time before T2 is created, creates T3. T3: Starts as T2. Trunks returns, Goku survives, they train to defeat the androids. Suddenly Cell appears and we got the Cell Saga. Cell is defeated, a stronger Trunks returns back to where he left, destroys the androids by power, then kills Cell before Cell can go back.
And future 17 and 18 are stronger because their reactors gives a permanent boost the longer they are alive, which is why they are stronger in Super compared to Z.
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