Daft Newbie question about widebody width

[parliboy]

So. I've got a copy of the Williams Widebody plans, which suggests that the width of the cabinet is 594mm.  And I've got this page, which says the width is 630cm.  And if I'm using 3/4 in. wood,  then that's 38mm, or about the difference between the two.

So does that mean that the Williams plans specify the inner dimensions, rather than the outer dimensions, or am I totally misunderstanding?

Thanks in advance.