Dr Stephen Strange accidentally opened up the Multiverse- Him and Peter Parker may have managed to pull everyone back to the right universe, however they have accidentally taken people who knew Peter Parker from their own universes, and pulled them through into the wrong universes. This is something that happened to Anastasia Garin, a young widow who Natasha Romonoff took under her wing, as she helped with a lot in the background of many Avenger fights. The young girl is now back in 2014, with no idea how or why she was there. Finding out the Avengers didn't exist, the population had never been halved, and there was no one else bar her from a different world.