Beating God of War isn't really that surprising.
Spider Man has always been a step above other comic book characters in terms of commercial success and popularity. I remember reading only a few years back that Spider Man merchandise was bringing in as much as Avengers, Batman and Superman combined. With the growth of the MCU driving it, including the huge hype behind Infinity War and other recent live-action Spider Man appearances actually being enjoyed by the consumer, this game was always gonna do well. Just like the Spider Man 2 game for PS2 did well, coming off the huge popularity of the Raimi films.
The same could be seen with The Force Awakens, the re-invigoration of the Star Wars franchise and Battlefront selling as well as it did (despite being not very good!)... I'd also argue that Batman Begins/The Dark Knight helped to push the Arkham franchise of games, by once again making Batman a mass-market media darling.
With the continued push in the MCU, this game actually being good and the continued popularity of the PS4, this will be hitting 10 Million in no time at all.