Headcanons

[Esfír Dedragón]

When Future Trunks went back in time to the Main Timeline, Dr. Gero saw a new powerful fighter that he'd never seen before and decided not only to create one more android, but to also modify himself as the last created android in order to deal with Future Trunks, just in case he would appear again.

Not possible. Dr. Gero specifically indicates that he not only stopped his own spying on Goku and the others after the battle with Vegeta on Earth (so from the Freeza arc onward, they weren't being observed), but he makes specific note that he's never seen Trunks before nor has any record of him in the files at all.

Oh, really?

Then, short of calling it a plot-hole, perhaps #19 and #20 did appear first but were defeated in a somewhat similar way in the Original Timeline and were not mentioned to Future Trunks when he was growing up, since #17 and #18 were a bigger threat.

[Darkprince410]

Oh, really?

Then, short of calling it a plot-hole, perhaps #19 and #20 did appear first but were defeated in a somewhat similar way in the Original Timeline and were not mentioned to Future Trunks when he was growing up, since #17 and #18 were a bigger threat.[/quote]

Dr. Gero stating they didn't have any need of spying on them after the battle with Vegeta.

[spoiler][/spoiler]

Dr. Gero indicating that he had no record of Trunks at all.

[spoiler][/spoiler]

In-universe, it's an unexplained plothole. Out of universe, it was due to Toriyama's former editor at the time insisting that Toriyama change the villains, as an "old geezer and a fatso" didn't make for good villains. He's also the reason #17 and #18 later got pushed aside for Cell.