It uses a simple algorithm to detect passives: be + past participle. It can't detect passives that don't follow that pattern, such as prepositional passives and

It uses a simple algorithm to detect passives: be + past participle. It can't detect passives that don't follow that pattern, such as prepositional passives and get passives, and it labels some clauses as passive when they aren't. Ironically, if you chose a random passage from Hemingway instead of Lincoln, it would litter that with spurious warnings too.