I think what you're seeing is a market that's trying to find a price. In other words, as you've observed, salaries really are all over the place.
Developers who are really desirable and have an information advantage and find the right company might be able to command the 190k salary. But you'd have to be really, really impressive to pull in that kind of coin *and* fortunate enough to come across a company that recognizes that's what it takes. And I don't just mean impressive as in "knows Rails inside and out". I also mean other soft skills like good leadership and ability to pull projects over the line at a rapid, sustainable pace.
The complication is that even though pure supply-demand economics suggests Rails developers should be "worth" the top end of that range, very few companies are willing to admit that yet. Many hiring companies are still thinking developers "should" be in the 80k-120k range like they were 1-2 years ago. So, those companies are more likely to hire a sub-par developer who is willing to work for 100k than a good developer who is asking for more, even though we all know that's a bad economic decision. You'll have to wait for those kinds of companies to work through a few bad hires (and the consequences) before they give up and spend what it takes (or cancel the project if the economics no longer make sense at 160k+).
If you're looking for a single-point number to hang your hat on, I'd say 140k is a good one. But bear in mind there's a huge +/- on either side of that.