Oh, you know what, you're right. I noticed when trying to take a photo that it wasn't as green, as I remembered, and now having pulled it out (that's what she said), the spine seems to have lost some color in comparison to all other sides indeed! Plus it was the first and oldest, I bought the other two much later. Damn...
Very curious situation indeed. Did the colour gradually change over time, or simply happen over night?? Maybe a hobbit even snuck in and replaced them all, I guess we'll never know!