But the idea is that the rules for PCs are different than the rules for monsters. If the intent is to make the enemies viable challenges for the PCs without giving the PCs insane amounts of treasure, accounting for every bit of the enemies' AC or attack bonus is going to be a nightmare. Honestly, I would just figure that the chainmail gives a +6 bonus to AC, and replacing it with scale mail would give a +7 bonus in its place. If you want the drow to have a better AC, give him one. 4E DM's supposedly don't have to justify every stat.
For example, look at the Elite and Solo Monsters section on pages 185-186 of the DMG, which instruct you to boost up to three defenses by +2, with no explanation of where those AC points are coming from. If you want a tougher monster, make a monster tougher.
I don't mean to sound rude in any way, but I think Archmage had it right. The difference between 3.x and 4E is that 3.x focused on using the rules to create challenging enemies, and 4E focuses on using challenging enemies to create the rules. It's more important to get the challenge right than get the rules right, because the game is about the challenges and the fun the characters have in overcoming them.
For example, look at the Elite and Solo Monsters section on pages 185-186 of the DMG, which instruct you to boost up to three defenses by +2, with no explanation of where those AC points are coming from. If you want a tougher monster, make a monster tougher.
I don't mean to sound rude in any way, but I think Archmage had it right. The difference between 3.x and 4E is that 3.x focused on using the rules to create challenging enemies, and 4E focuses on using challenging enemies to create the rules. It's more important to get the challenge right than get the rules right, because the game is about the challenges and the fun the characters have in overcoming them.