Marner's RFA deal was laughable. Everyone made fun of it at the time and since it was signed Marner has done nothing to dispel the notion that he's not a player you can win with. It should have little bearing on his current deal.
Getting a lower average on your first UFA contract is rare but this could be a case where it happens. I don't know too many teams that will be comfortable throwing more than 12M at a tiny winger who's current team is seemingly prepared to let walk for nothing.
People were talking about Gaudreau (RIP) getting more than 10-11M as a UFA, but the demand for him around the league wasn't as high as expected given his point totals. He ended up signing for under 10M in Columbus, widely considered an undesirable market for most players. Marner could be in for a similar treatment.
Rantanen was about to be a UFA and the contract he got from Dallas largely reflected his value on the open market. There may have been teams willing to offer more, but Dallas is an extremely attractive destination for players. So is Vegas, so my point stands that they could probably get Marner cheaper than another team.
I'd be willing to bet that Marner's salary next season will be less than 12.7M, and I don't expect teams like Anaheim, Chicago or San Jose being all that interested in bringing him in.