ISBN mixup

When I ordered Clouds of the World: A Complete Colour Encyclopedia, I thought I was ordering this -

 It's a large book, found in the library, published in 1972 - I liked the pre-digital photos and the good explanations.

What arrived is this -

Looking on the invoice, I found this description: "Clouds of the World: A Complete Colour Encyclopedia (The Islands series)" -- hmm, somehow that escaped me till then. Quick email to the bookseller, 2nd Look Books, revealed that both books have the same ISBN!

How could that be? Aren't ISBNs supposed to be unique numbers to identify books? Not in 1972 ... that was the year that the international standard book number system was imposed; before that, the US and the UK had their own book numbering systems. "The Falkland Islands" was published in both the US and UK, by different publishers ... so one of the numbers given to it happened to be the ISBN of the Clouds book - and it's by ISBNs that booksellers keep track of books.

2nd Look Books suggested I keep the book, or donate it, rather than return it  - and they refunded my purchase price and postage. Good service!