My guess is because “little” is distinctively different from most of the other early words that they are introduced to. The words “something” “little” “beautiful”….. as whole words are very unique when compared to The rat ran and sat on a mat. “Big” and “bug” as whole words are very similar. I notice that my children sometimes mix up there, here, and where because of the similarities. I know that my oldest child hated Bob books, Alphaphonics and other similar readers. He loved the story line of Dick and Jane, Treadwell Readers, and the Ginn readers. I used some Poetry and nursery Ryhmes in a Charlotte Mason approach as well.
Now where they are at I just sit and listen to them read and then remind them some of the rules. It is now one of my favorite things about homeschooling.