Not always. Different operating systems and browser combos have different ways of rendering fonts. I've seen fonts that look good in OSX in Firefox look poorly in Windows with Firefox (Gentium in my case).

Also these images look like they used gray scale instead of sub-pixel rendering so the differences in pixel geometry should be more or less mitigated.

I'm sure in theory it could look better as actual text for certain setups, but this probably allows them to get the best looking text in the widest amount of configurations.

Images can be rendered at the subpixel level if they are oversampled (e.g. "retina" image on a non-retina screen). Browsers just don't.