You're not wrong. This is the expanded version of the story. See, when chapter twelve was first published in single issues, it was supposed to be only fifteen pages long, like most of the other chapters. Rosa, however, thought that the story was too big for fifteen pages and managed to nag the publisher into expanding into giving it a second page...so the first published version of the story is just sixteen pages.

When the time came for re-publishing the story for the collected trade, Rosa got to expand the story a little more, bringing the page count up to nineteen. The three new pages were added as extra rows in certain places, usually for expanding on a situation or add an extra Barks reference. (The fight with the Beagle Boys is where the expansion gets most noticable,as there was more room for the action to develop.)

If you look at the two "extra" rows in these scans, you'll see that the panel borders don't match completely with the rest -- the line is slightly thinner. It's because these rows were drawn much later.

(For those of you who wonder, the two extra rows in question are: The one where Donald points out the Eisner award, and the one where Scrooge kicks Donald. Neither of these scenes happened in the chapter's original prining.)