A margin image is a float whose whole width has been pulled out of the column, so it takes no room in the line at all. Read this paragraph and the one below it against the float above: those lines shorten around the picture, and these do not. The measure is exactly what it would be on a page with no picture at all.

That makes it the arrangement for a picture that supports the reading rather than interrupting it — a reference, a specimen, a piece of evidence the reader glances at and returns from without losing their place in the sentence.

It needs a margin to sit in, which means it needs a band wider than its column. In the documentation column there is no such room, so what you see here is the fallback; open the full page preview to see the picture take the margin.