Ross explained it in his last video - there are reasons to be skeptical and unsure if it’s truly there until at least 1.4 mil signatures. And more votes is never bad. So both need more attention. If it reaches people in the EU it will also reach those in the UK.
There’s always just people that mess up on the form. But they also monitor the sign rate and saw some periods of higher than normal signing in the middle of the night in the EU - indicating someone might have ran a bot to sign with invalid information. The EU only validates the signatures once the petition is closed, so they need a safe margin where even with a significant amount of invalid signatures, they still make it. Afaik 1.2 mil is about what they would expect for a normal vote of this size to be safe, and 1.4 mil is basically more than enough to compensate for any bad actors.