All the reincarnators were “conceived” at the same time. But Kumoko’s spider egg only took a month or two before it hatched, while her classmates who became humans took nine months to be born. So while they’re still forming in their pregnant mothers, Kumoko has been running around the Labyrinth gaining XP and constantly evolving. She’s about to encounter her first classmate, the infant now known as Sophia, who’s been reborn to a noble family (the ones being attacked in the carriage).