Perhaps the extra takes place in between them getting together and the last part.
http://www.mangago.zone/read-manga/goodbye_lilac/mf/fd4ccf252bd2c464/Vol1_Ch5/pg-46/
I suppose... I didn't think of that since the atmosphere between them during those last pages of the last chapter compared to the extra seemed "fresh" even though some time should've passed... Oh well, I was reading into it too much. I'm just glad they got to be together eventually. I was really worried at the start especially since sensei had that sort of personality even though Katou was the one coming onto him. Don't get me wrong, I respect that side of sensei. I was just worried he'd ultimately reject Katou and I'd end up crying with another of the mangaka's work (That other one is entitled Tamayura)
Tamayura had a happy ending too though.
it did, it did...but I cried! It was bitter sweet! （/TДT)/
wait...
from here:
http://www.mangago.zone/read-manga/goodbye_lilac/mf/tr_chapter-5.5/pg-2/
...I thought they already told sensei's father at least??
http://www.mangago.zone/read-manga/goodbye_lilac/mf/fd4ccf252bd2c464/Vol1_Ch5/pg-51/