[dgatwood]

IIRC, most of those Kindle readers, by default, set the line height to 1.2, and that's the minimum allowed line height, so the minimum distance between lines is 1.2 times the em square. I don't think it cares about the line gap, though IIRC some readers (those using the non-QT version of WebKit, like iOS) do take it into account if the em square plus that value exceed 1.2x the em square. I think.

In the photo above, the em square is probably smaller than the sum of the ascender and descender, resulting in the line height being too small.

I've seen this frequently, mostly with older fonts. I haven't seen it in any fonts created in the past decade or so, because most of the font tools these days provide access to parts of Adobe's font SDK, which IIRC provides (among other things) tools for computing all of those values automatically.