One of the most memorable storylines in Dragon Ball Z was the Android Saga, as the wicked Doctor Gero unleashed the deadly Androids 17 and 18 to kill Goku. While the Z Fighters were warned three years in advance of this by the time-traveling Super Saiyan Trunks from an alternate timeline, Future Trunks was shocked to discover that the Androids he faced alongside the Z Fighters were far more powerful than the ones he faced in his own timeline. Here's why the Androids from the prime Dragon Ball timeline may have been noticeably stronger than the ones Future Trunks previously encountered.
Returning to the prime timeline after defending his home for three years, Future Trunks finds plenty of unexpected surprises awaiting him. The Androids whom the Z Fighters initially confront, Androids 19 and 20, do not resemble the ones terrorizing his timeline, Goku is stricken by the rare heart virus later than anticipated, and Androids 17 and 18 appear to be far more powerful than the ones Trunks is accustomed to facing.
For much of his time in the prime timeline, Future Trunks openly ponders if his tampering with history has caused these severe divergences. However, what's probably the cause is a mix between the changes time-travel has caused to the space-time continuum and Trunks' own ignorance of the cybernetic opponents he has feared his entire life.
For starters, it is absolutely clear that Future Trunks arriving in the main timeline did in fact cause changes to the fabric of reality that weren't present in his own future. From the emergence of Cell to discrepancies with Goku's heart virus, the history Trunks knew did change from his divergent timeline. This could mean that the alterations to reality could have indeed made Androids 17 and 18 significantly stronger, with 17 now as strong as Piccolo -- even after the Namekian fused with Kami to become vastly more powerful than the Piccolo of Future Trunks' timeline. And as the Dragon Ball franchise progressed, the Androids would continue to increase their power further, with 17 roughly on par with a Super Saiyan Blue during Dragon Ball Super.
The other major factor for Future Trunks being surprised by the Androids' strength in the prime timeline is that he previously underestimated the power of the ones from his alternate future. This miscalculation fatally extended to Trunks' mentor, an older version of Gohan, who learned that the Androids of the alternate future had secretly been toying with him all along, before unveiling their full strength to kill him.
Trunks likely also misjudged his two opponents, unaware of their true power before traveling to save the prime timeline. With Goku still alive there, this reality's Androids were less apt to toy with their opponents in order to find their true target.
The Androids in the prime timeline are indeed stronger than the ones from Future Trunks' divergent timeline, but Trunks also wasn't quite aware of how powerful his longtime adversaries were, making the disparity seem all the more glaring. Ultimately, Trunks would easily destroy the Androids from his timeline after returning stronger than ever, while the Androids from the prime timeline would grow even more powerful themselves. And given the amount of times the Z Fighters have traveled through time in Dragon Ball Super, there could be untold fluctuations caused to the prime timeline by tampering with the course of history.
