The fact that this is a 'second edition' under the same ISBN is indeed rare.
My favorite example of this is Game of Thrones, which has been the same edition and same ISBN since 1996, despite dozens of reprints, and at least four changes to the boards and dust jacket.
So, the minor changes they made could totally have justified making this a 'Second Printing,' including changing the cover, but making it a second edition is certainly unusual, especially with the ISBN.
However, I think they did so just to make it blatantly obvious for collectors who care about it and had purchased the first edition, first printing already as a collectible rather than as a strategy guide. They left the ISBN the same so you could find and buy it easier, meaning it is really more of a second printing than second edition in fact, if not in name.
It is also possible that the first edition didn't include a number line at all, so making a second edition rather than a shortened number line makes it easier to distinguish.