I suspect that the human graders were the biased ones, and that this automated test is more accurate. Schools frequently inflate test results when given the opportunity (especially when low results reflect poorly on the school).
How do students known to be fluent in English do on it? Do they pass reliably?
Edit: Here's a discussion of a similar phenomenon in the context of high-school graduation rates. Graduation rates regularly go up by a very large amount when standardized tests stop being required, but that's not because otherwise-qualified students were doing poorly on standardized tests.