Head First Mobile Web possible error?

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This is a question regarding Head First Mobile Web:
I'm looking at p.16 of the printed book and the last css rule on the page:
#main {
margin: 10px 260px 0 250px;
width: 460px;
}
I don't see how it could be 460 pixels wide and still maintain a 10px gutter between it and the next column. If each column has a 10px margin (according to an earlier css rule), then to fill the 960px body width, I count the following:

240px column + 10px margin + 450px column + 10px margin + 240px column + 10px margin = 960px.

That is, the width of #main is not 460px but 450px (empirically, it also looks like the 2nd gutter of 10px is maintained if the width property is left out altogether).
Owing to my not-so-expert knowledge of HTML/CSS I could be missing something, in which case it would be instructive if someone could set me straight.