Subject lines… the easiest thing to write, the hardest thing to get right. There is just no single rule, tactic or word that works every time. Best practices sometimes fail and A/B testing can produce unexpected results.
Why is this? In simplest terms, being original keeps your subscribers on their toes and can spark in-actives into life. But if every subject line you write is surprising and original, your subscribers catch on and that lack of clarity has a negative effect.