A researcher after my own heart - love that you're really trying to understand!
I have a few thoughts - first, I'm not sure you can compare the 4XX to the 400i in the same 1:1 way that you rightly can with the 6XX and the 650. It's been a long time since I bought mine (first run), but I seem to remember them saying that it wasn't quite the same as either the 400s or the 400i. It's been a while though, so check that.
Second, I love Rtings approach - trying to bring objective measurements to the forefront of reviews. In fact they were a major deciding factor in my purchase of my LG OLED tv last year. They rated it as the number one option for consumers at the time, and I found that everything they said about TVs was pretty much spot on when I tested them. However, that hasn't been my experience with their headphone reviews. In fact, I've had a lot of "Huh?" moments in looking at their reviews compared to my subjective experience. Doesn't mean they are wrong, or that you shouldn't consider them - that's just my experience.
Ultimately, I think it comes down to the much broader variance in individuals' subjective acoustic experience, vs. say video. In short, I'd say it comes down to the fact that the artificial head they are using looks nothing like mine, and isn't going to interact with sound the same way mine will. I'm just not sure their approach works in the real world. Still, I applaud their effort, and you should give their opinion its due weight.
Last but not least, there are so many variables that I could never fully articulate - the test songs I use , the variance in what's driving the headphones (usually a Valhalla 2 for the 6XX and a Magni 2 for the 4XX), the shape of my head, my age, etc. Think of me - or any reviewer - as one data point, nothing more. I stand by my experience, but doesn't mean it will be the same as yours.
Finally, it's worth mentioning that neither of those two headphones are really standouts on soundstage, and not even the best headphones can compare to decent speakers, set up correctly, on that front. There are much cheaper headphones (e.g., my HD558 and Philips SHP9500s) that I think are better on the soundstage front (though not overall).
Hope that helps, and remember that I don't think you can go wrong with either choice!