It is worth noting that not all iPhones are bought on day 1. 6s was discontinued September 12, 2018 (https://en.m.wikipedia.org/wiki/IPhone_6S) and customers may have bought these phones later.
The 6S has 2GB of RAM versus the 1GB of the 6, probably the principle differentiator here as otherwise the specs look pretty comparable (6S also has 2 more GPU cores and a faster CPU clock rate which may be other contributing factors).
Looks like they were still being manufactured and sold in some regions until at least August 2019.
https://arstechnica.com/gadgets/2019/10/apples-new-repair-pr...