Not only do some years in American schools have weird names that no-one else understands but us, we also have different grade numbers from the rest of the world. Non-Americans can get pretty confused by the whole system we have.
Part of the problem might be the place that all the counting begins. We start from Kindergarten, not 1st grade really, which puts us off by one before we even begin. Other countries sometimes start school later as well, which definitely won’t help everyone work out what’s going on.