There is no dictionary to check against, because none of the spellings is wrong. So rather than asking whether a word is correct, the check asks whether two words look like attempts at the same word, by how few edits separate them.
The threshold scales with length, and that detail is what makes it usable. One edit in a three-letter word is a much bigger change than one edit in an eight-letter word, so short words are held to a tighter limit. It is what keeps hai and hua apart, two edits and genuinely different words, while grouping kya and kia, one edit and the same word.
English words are skipped entirely. English spelling is settled, so English has no variants to reconcile and comparing it only produces false pairs: there with their, same with some, bat with but. The cost is a miss on a romanized word that happens to be spelled like an English one, such as main or hum. That is the right way round. A false alarm on a correctly spelled word spends your attention for nothing, while a miss only leaves work undone.