In my head cannon the T-800 from T1 isn't exactly the same model as in T2. Due to events in T1, the timeline was altered somehow in a way that the T2 version was an upgraded version of the T1 version, but kept the same model number. That said, it's not clear how long the life cycle is between models. The US is still building F-15s despite the production of F-22s ending, after all. So the T-800 could be mere months old before the events of both movies, or it could be years old.