I think you're overgeneralizing from your own personal experience. For the record, my units were 5 levels lower than yours when I beat Chapter I, and I'm sure Raven can probably do it at Level 18. It all depends on how much care you put into analyzing the enemy.
I didn't read this massive textwall of a thread I came back to yet but I saw this.
I beat Chapter 1 of Jot5 (in the 1.0 release state, which was noticeably harder than the current version) with a party level average of 9-10 after Ganon Part 2, without performing any nonessential battles for Gil or JP or drawing out any essential battles for farming purposes, without ever entering the Job Change screen (meaning I only ever had Base+Chemist), without using Move-Find Item, without using the Soldier Office, buying every skill in the order of cheapest to most expensive at the first possible opportunity, and with only 1 reset on Ganon 2.
Yes, I made
the damn game (at least in this regard), but uh, yeah. Most video playthroughs (and non-video playthroughs that I've been made aware of) come out at about Level 15 at the end of Chapter 1. 20 seems to be the upper-end if you've been grinding.