Spoiler alert for those who haven’t yet managed to catch it in theatres…
It has to do, of course, with the use of time travel in the movie, and specifically the amount of Pym particles that are shared by the characters. You might remember, that the Avengers only have a limited amount of Pym particles to help them time travel through the Quantum Realm and into the past to gather the Infinity Stones prior to Thanos’ snap.
Ant-Man insists numerous times that the assembled heroes only have enough particles for one round trip each. They go through two test runs, one for Ant-Man and another for Hawkeye, which leaves enough particles for the Hulk, Iron Man, Captain America, Black Widow, Rocket, Hawkeye, Thor, War Machine, Ant-Man, and Nebula each.
Although Tony and Steve travel back to the 1970s to get more Pym particles, Nebula definitely doesn’t, and when she gets captured by her past self and a version of Thanos from 2014, the old Nebula then uses the new Nebula’s particles to travel into the future at the behest of Thanos; meaning that both Nebulas have now used up their only round trip.
This creates a plot hole: how does 2014-Nebula pull 2014-Thanos into 2023 for the final battle when she’s already used up her Pym particles? We see Thanos’ ship flying through the quantum realm, but we don’t see him get any particles of his own.
One possible answer is that thanks to Thanos’ alien technology, his scientists were able to reverse-engineer Pym particles of their own. But it still doesn’t explain how Thanos was able to do this so quickly.
However, this isn’t the only intriguing detail from the movie that we’ve covered. Check out this article we penned, which looks at the subtle reference to Iron Man 3 that you probably missed on your first viewing.