I wondered that also, I decided our kids are smart enough to figure it out but it grated on my nerves every time I saw it, I considered editing it but basically I am lazy
so I left it alone and what do you know.. The first word my son actually read to me from little reader was the word " blue" and it was in a different colour print...red from memory! So obviously he is smarter than me!
I also noticed in common animals a bird and a duck are listed separately, yet a duck IS a bird!