Board Thread:Questions and Answers/@comment-5503727-20140511180736/@comment-4848837-20140511193745

The Gwen 10 timeline and the timeline from the original series episode of the same name are totally different. The former is an alternate version of the main timeline, where instead of Ben finding the Omnitrix, Gwen did. Likely, a metric ton of differences between the two timelines spawned from that. In the "What If?" episode from the original series, we saw Gwen get the Omnitrix, but for some reason, Ben had all the memories from the main timeline.

We don't quite know what made this happen (was Ben sent to another timeline? was the Ben of that timeline's memories copied from the other? did Ben go back in time in a way that affected the timeline so much it caused the Gwen 10 timeline (kinda)? who knows?), but the fact that Ben knew so much made a big difference. In addition, Vilgax's ship never crashed, also spawning a boat load of differences.

So basically, Gwen 10 timeline: like the main, but Gwen got it instead of Ben (but Ben didn't know about the Omnitrix like in that OS ep); that timeline from the OS: like Gwen 10, but Vilgax never crashed, Ben had his memories from the main, and a lot of other things changed from there.