One might reasonably ask a different question: Why are American kids not so good at spelling?
The answer is, that learning in general, and spelling in particular, is not valued in this culture as it is elsewhere.
Show some signs of excellence in this area or that, and you're branded as "weird". Kids, particularly school-age kids, try to bring others down to their own level, rather than elevate themselves. That carries on into adulthood, as well.
Gubmint skools lead the effort in that regard.
Anyone know if this girl was home schooled?
Socialism 101. Teach socialism in schools through experience and by living it. It's the best way to learn.