Ironically that variability is good. As long as the bottom bound is high enough to ensure decent progression or the player is given some sort of agency or safeguard allowing them to keep progression at a decent level. People play alot longer when things are highly variable vs when they are much more homogeneous. The times where you get the towns without the crack a book or mailboxes or etc are what make the times you DO get them feel so good.
This has alot to do with psychology and the methodologies of reinforcing habits.
Variable Ratio Schedule of Reinforcement is heinously effective. tl;dr uncertain rewards. But you do need a minimum success rate of returns to maximize it. See casinos and the "wins" where you get your money back alot today vs just losing all the time like it used to be. It's well known and used constantly in gaming, though it's hard to find good publicly available examples. Talking about that kinda @%$# publicly just has a bad look so game dev normally tucks that one away in back corners in recording limited lectures and meetings never intended to see the light of public. The Jimquisition has covered it a few times and snuck away or dug up some footage before though

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Essentially as long as there are sufficient protections against being RNG screwed you're good. And one thing I've learned about this update is that you kinda do wanna focus if you want to progress fast in an area because the loot pools get in each other's way. You'll see this really take effect when you start completing magazine collections of stuff you've perked in, other perked stuff speeds up. On the other hand if you're jack of all trades you're master of none for quite some time. So I think alot of issues have to do with people spreading themselves too thin.