I see what you're getting at, but generally speaking I'm not ranking in terms of objective perfection, firstly because nothing's perfect, and secondly because I can't claim to be some authority on what's objectively good. I can only talk about my response to a film, and how much I enjoyed it/didn't enjoy it and why. And so if I absolutely loved a film, it has earned its place amongst my all-time favourite films, and I can't imagine loving it more, then of course I will give it a 10.
My rating system, when I review films on Letterboxd, goes by the 5 star system, and here's the criteria I go by:
***** - Classic
****1/2 - Excellent
**** - Great
***1/2 - Very Good
*** - Good
**1/2 - Okay
** - Mediocre
*1/2 - Bad
* - Very Bad
1/2 - Wretched Turd
Translating that to the 10 point rating system means that Captain America: The Winter Soldier - by my ranking a 4 and a half star film - becomes a 9/10 movie. I'd say that's a fair reflection of my response to the movie, as someone who feels it's already one of the year's best. Meanwhile, your 6/10 rating translates to a *** rating, which is "Good," meaning that overall you liked it but there were a lot of flaws that hampered your enjoyment or you didn't think it was anything particularly special. I think that seems to largely reflect your own review that you wrote earlier.
But when someone says the film is a 3/10, that by my ranking is a *1/2 star film. That's the rating I gave Sweet Home Alabama, Swimfan and Priest. That's pretty dire, and based on the actual flaws the Crave reviewer laid out in his review, it seems the score is much harsher than what he actually found wrong with it.