Was that with mods? I've seen quite a few diamond-armored mobs in this world but only about 5-6 in more than 2,000 hours of gameplay, with close to 160,000 mobs killed, perhaps 90% of those hostile (more recently I've been killing animals for trading but most of the mobs I kill are still hostile; they did not add stats for specific mobs until 1.7); the last time I saw one was several months ago, and before that, another two months earlier. One time I did see two about 30 minutes apart though. For perspective, the chance of getting diamond armor is one in 2,333 mobs with armor, or one in 15,551 zombies and skeletons at the maximum regional difficulty.
This is on Normal difficulty but I modified the regional difficulty calculation so it starts at 50% of the maximum so it only takes half as long to reach the maximum (this does not affect the save files as it simply modifies how the game converts the inhabited time to a difficulty so as I explore new areas they do not start out from 0; the area around my main base is probably already at the maximum even if I do not spend much time there and the world was generated in 1.5, when the game did not save it).
There's also a difference in how the game calculates regional difficulty across versions; prior 1.8 the chance of regional difficulty-dependent effects linearly increases from 0% while since 1.8 it must reach at least 2 (as seen in the debug screen) to have any affect (this means that on Easy, which never exceeds 1.5, mobs will never spawn with armor in 1.8 or later, but I've seen armored mobs on Easy immediately after creating a new world in 1.6 or 1.7). Also, prior to 1.8 the chances of effects can get higher on Hard since 1.8 placed a cap on the calculation (anything over 4, the maximum on Normal, has no further effect); the chance of armor is up to 15% on Normal but up to 22.5% on Hard, compared to 15% for both in 1.8. On the other hand, since 1.8 the total time played is also a factor so you don't need to load a chunk for 50 hours to reach the maximum inhabited time-based difficulty.
I was playing on hard mode, using 1.6.4. 1.6.4 Using Your first world mod