http://www.gdcvault.com/play/1012186/The-Psychology-of-Game-...
More often than not, true RNG in game design takes a back seat to fun.
Basically, the higher your chance to hit with an attack was, the the sooner it would break a streak of misses and force a hit. This is definitely not random, but seems to be more satisfying to the player. Plus it no doubt cuts down on the number of complaints on the forums about "unfair" RNG.
They could do with learning a little from other games and making it a little less random.
Ultimately you can see it as a bug in human reasoning but we're the ones playing the game.
If every house had one pistol and one rifle it would get pretty dull. Raiding 3 houses and only getting a crossbow, bandages, and SMG magazines is frustrating but it's exciting. Just last night I got to #3 or #4 with a crossbow and a pistol.