AR was mentioned by a previous poster and that's totally what I was thinking. There are kids who will "sandbag" their AR stuff so that they get the higher scores without much work. When I was a kid, we had a program at my elementary school called Book It. It had different levels with different prizes and you tracked your reading based on the amount you read...either by number of books or number of pages when you were reading chapter books. You had to read 15 books to move up to the next level. I was in 4th grade and several of my friends were reading 15 "books" in a night because they were reading board books. I can totally see a parent who has a kid doing that type of thing requiring that the kid get at least one book at reading level, if not several.
In addition, if the mom or the library has a limit of how many books they can check out at once, I can certainly understand a mom wanting her kid to pick out one or more books that will take more than 10 minutes for the kid to read.
Unless you are seeing the kids, and books and seeing the moms and kids regularly, I wouldn't really assume anything. You never know if they have been working on branching out or if the kids have been trying to "cheat" reading programs or whatever.