They are ignoring everything, as technically he goes back to the beginning of the first movie, but the movie we saw and he is expecting to go into, is altered, due to the fact a terminator in the continuity of the originals, 1-4 was sent back to when she was born.
I guess you could call the movies 'chapters' and chapter 5 is missing from the film series as this is when a terminator is sent back to the birth or sarah connor. Then we have this film which is i guess chapter 6, which shows the new time line. Confusing i know