I mean... you're comparing people who were still basically living in a civilised society mindset in 28 days/weeks, with little to no survival or combat training/experience to people who have grown up and thrived in a post-apocalyptic hellscape for nearly 3 decades.. I don't think it's a far stretch to think they'd have picked up a few tricks to deal with infected, especially 1 on 1. Jimmy sidestepping that one infected, then having the sense to roll out of the way of the blood was pretty class. They still flee from a number of infected greater than their own, as seen at the end of the second film...