In short, we see so many market inefficiencies in hiring, it'd be hilarious if it weren't folks' lives we were talking about. There's no practical difference between someone who has 4.750 years of experience and 5.250 years of experience, but the market dramatically prefers the latter.
Waterluvian commented below about the hiring markets self-optimizing themselves to efficiency.
https://news.ycombinator.com/item?id=16702612
In short, we see so many market inefficiencies in hiring, it'd be hilarious if it weren't folks' lives we were talking about. There's no practical difference between someone who has 4.750 years of experience and 5.250 years of experience, but the market dramatically prefers the latter.