Board Thread:Questions and Answers/@comment-90.221.228.185-20150511233505/@comment-28251580-20160808142749

16.5 was never a part of Volume 1, hence it wasn't omitted, but never there in the first place. When SAO was translated at baka-tsuki, some people added Ence's translation into the PDF, but it was never a part of the released volume or even the Web Novel. It was on a hidden section on his website.

You can read tha baka-tsuki discussion for yourself here and here