@manga4joe
I believe that the numbering listed on the last page is actually referring to the issue of Harem that it was published in. It says "ハレム Vol. 9 収録", meaning "Harem Vol.9" and the last word I get a definition for of "recording; compilation" which makes sense in context but doesn't translate easily and probably is being used in a common way that I'm just unfamiliar with . Even in the raws the title page will show a different Chapter number. This also gets a little weird as Chapters 9 and 10 (in raws, Ch. 10 "I Want You" and Ch. 7 "The Two Alone, at the School at Night" here) both list Vol. 8. Presumably they were both published in Harem #8. That matches what I was able to look up in the contents page for Harem Vol. 8 where it states NtK being on both pages 57 and 87 and labeled as "2本立て", roughly "double feature".
But you're right. There was originally another Tachi-focused chapter in between "Future Is By Your Side" and "The Choice Is Yours". In the original release it was Chapter 13 with "Future" as 12 and "Choice" as 14. While my Japanese isn't very good, the title of that was ""気持ちいいから", and based on my reading and the Vol. 3 Contents page posted with Chapter 13, that's "Because It Feels Good" which is collected as Chapter 16. Based on the titles it looks like Volume 3 changes it up to: 14, 18, 17, 13, 15, 16 in the original publication order. Considering the content of the chapters it seems likely that the titles were simply swapped for 17 and 18 because they directly follow one another, so switching their order around wouldn't make any sense.
So no worries. Nothing was being skipped over. Just the usual shuffling of chapters for the collections and irregularities in the publishing. It's just a reference to what volume of the magazine it was originally featured in.