Reaver Ghouls and Roamer ghouls are total crap. Again Rotting corpses don't get stronger! Second ghoulification is not an instant thing (please for the love of god don't bring up Moria) The armour does not become apart of thier body. Reavers were added because beth need stronger enemies, they wanted bigger, dumber bullet sponges. Why did then not just make the enemies smarter? I will never know.
It's part of how Bethesda creates enemies in their games (the ones that Bethesda Game Studios creates, not the crap that gamesas published like "Wet" and "Rogue Warrior"). For instance in Oblivion we had several different types of the Dremora, a demonic humanoid race that were the primary enemies in the Deadlands/"Realms of Oblivion" (I say Deadlands because the proper name for the realm you went to in Oblivion was called the Deadlands, even though Bethesda for some reason just instead called them "Gates to Oblivion"). With an RPG like Oblivion or Fallout 3 you have two ways of dealing with enemies:
1) Have them level up with the player
2) Create different sub-creatures that correspond with the player's level.
In Bethesda's case they utilize the second option because the first option irritates most fans. "Why am I level 20 and these super mutants are still so hard to kill?!" Well it's because the super mutants are also
level 20 now. If instead it's just "super mutant", "super mutant brute", etc. it is less aggravating as now the fans can know which super mutants will be easy to kill (the low level, generic one), and which ones will be harder to kill (brute, master, or the dreaded overlord), allowing them to pick and choose fights. I'm not much of a fan of that approach, but it's what makes a lot of people happy and what works.
And with the roamers and reavers the easiest explanation is that "they have armor on". Plus the ghouls of the east coast, or at least the Capital Wasteland may have had it a bit differently. With more direct hits (i.e. the White House), the flash of the bombs may have been the reason they have armor on. Rather than just having their skin rotting off through necrosis, their flesh may have literally melted and bonded with the armor. That's the best reasoning I can think of (well for the roamers. The reavers I just try to forget ever existed).