There's a few ways to look at the data. One is that it's harder to move to the top quintile in America than some other countries, and people perceive it's much easier than it in in America compared to those places.

The other is that it's much easier to get out of the lower quintile in America than those same countries.

There's a couple reasons that might be. One is that social mobility is actually higher in America, but there's a ceiling over which it's hard to overcome unless you're born there. Whether that ceiling is immediately under the top quintile or somewhere lower is an interesting question.

Edit: as pointed out, I actually misread the first graph, which is probability of remaining in lowest quintile. So, yeah, data looks like all bad news to me (unless you count those people knowing fairly accurately how fucked they are as a good thing).