Yeah, my best guess is that they used a lot of complicated math that the average DM isn't going to be understand. They probably figured out the most fun percentage chance to hit by examining levels 4-14 in 3rd Edition as well as playtested with different numbers (30% chance to hit the average enemy, 40%, 50%, etc).
Then, once they had a duration of combat that felt right and a percent chance to hit that was the most fun for the players, they figured out how many hitpoints enemies needed in order to survive the damage the players were doing for exactly how long they wanted the battle to last.
This required precisely setting the hitpoints of the enemies rather that rolling them from hit dice. It also required removing abilities like DR which make predicting the average damage per round of PCs very difficult when some of them might bypass DR while others don't. Also, some PCs are doing their damage with one attack, others with 3. DR makes it much worse for one of them.
Then on top of that, you have to keep in mind that the average encounter has 4 or 5 enemies...and that each of them likely has a different role. So that the damage that one enemy does makes up for lower damage by one of the others, etc.
So, given that most DMs are not going to understand or want to do this math in order to create a new creature, it is likely we'll get a table with the results of the math done for us. You'll likely see a chart with average numbers for each role of enemies. However, I still think that each monster will be created individually. It's a matter of knowing what the "correct" numbers are for that level and role and then modifying the individual creature to be more unique. However, that part I expect to be more art than science. You just have to know that dropping a monsters AC by 10 points and upping its damage by 10 is not an equal tradeoff. That's the sort of thing you just have to know the system really well to do, though. I anticipate that using stock monsters will be fairly standard for most DMs anyways.
