Because it's not really about the color, but it's about the undertones or the color behind the color. And oh, the places those undertones can take a color!
If this is a bit confusing for you as well, start with whites. In Benjamin Moore's off-white collection (really whites with different undertones), there are 140 tones. That's a lot of shades of white!
But it's all in the undertones . . .
Let's compare three whites and I think you'll see what I mean. Start with Benjamin Moore's Color of the Year, Simply White. It's close to a pure white, but against the white background of the blog, a bit of a yellow undertone shows through.
The next white is actually called Owl Gray. Yes, the name gives part of it away, but it's not just gray you see as an undertone but a bit of green as well.
And the final white is Benjamin Moore's Chantilly Lace. A crisp white with a - can you see it? - touch of an icy blue-green.
It should start to become apparent (yes, I understand monitors may be creating a bit of havoc) that undertones are an important consideration when selecting colors for our homes. Fan decks can be very helpful because they are ordered by undertones. (The greens go to the blue-greens then to the blues for example.) Or just put a color next to another color to see if they "go together". If they do, it's probably because they share undertones. If you begin paying careful attention, before you know it, all of your color choices will be the right ones!