Other factors- there are fewer mods and there is no Ed around to zap the spam. Spam that comes in during the day ususally disappears immediately, so we don't see a lot of it unless we are on here all the time.
Most spam never makes Latest Posts though and goes under the radar- there are usually a dozen or so posts, mainly in the early hours, from spammers who trawl old threads from say 2009 and post spam relies- usually threads about SEO, phones, flower deliveries, wedding dresses etc. Having said that i think last night was pretty quiet on that from too.