I ended up contacting Osidetiger , and got the definitive explanation. Here is what they said:
"The factory injectors use that lower seal to seal off a cavity that is part of the intake manifold. That part of the manifold has a port that introduces air at certain rpms. That air enters the injector tip in an effort to improve with fuel atomization. The factory injectors have small air ports molded in to allow the air to mix with fuel during operation. This was an attempt to get decent atomization out of a single hole (at the very tip) of the injector. The injector tip on the factory injector you can see is just a two way diverter and does nothing for atomization. The new modern injectors do away with all of this by using 12 atomizing holes instead. The additional air that enters the cavity in the intake bypasses the injector tips so the engine and sensors don’t see a change."
Looking at the injector mounting pocket in the manifold, the vacuum sealing surface is near the top, and is sealed by that large seal that both injectors have. Below that is that aux air passage that they mentioned, from which air comes out at certain RPM and tries to push fuel through the injector to better atomize it.
That lower o-ring on the original injector is therefore only used to prevent this auxiliary air from going
around the injector tip, forcing the air to go
through the injector. So the o-ring has nothing to do with maintaining a vacuum seal, which is why it's not needed in the upgraded injector.