I agree, it would be easier. But if that's how their family works- everyone gets an equal amount- it's not fair to penalize her for having a smaller family. Also, I'm sure it helped that Kody was able to be on her application but I think that's irrelevant here. Christine is a SAHM. Janelle works. Christine had no problem qualifying while Janelle barely qualified. They each have 6 children. I think it would be unfair for either of them to have to sacrifice because the other was struggling financially.
I'm sure there are other factors that need to be considered, but regardless, I don't think it's fair to punish Meri for having a smaller family- especially since it's not by choice.
ETA: I meant to quote CiderWench