I figured that, since Google must read every word of every page to spider it, they must thereby have, as a byproduct, the world's most accurate database of word usage frequencies. "Reading level" would then just be a measure of the average frequency of all the words on a page (thus making words learned in a first year ESL class simple, and technical jargon advanced—quite the same as the measure of difficulty used by language proficiency exams.) The fact that the average of Simple English Wikipedia articles seems to be more intermediate than basic, though (29/52/17), would argue against that—unless the calculations are being biased by all the very infrequent proper nouns.