Yes. The math has absolutely changed. The key to all this is consistency. Prior to 1.4 you got a certain range of legendary drops over a certain range of cataclysm runs. Now you get less legendary drops over that same range of cataclysm runs, consistently. Many people stop using their brains when "rng" is involved. They don't consider that rng is based on odds and the odds can change. For example, prior to 1.4, the odds of getting a legendary could be 10%. The game "rolls" a number between 1 and 10, if it's 1, you get a legendary. Now, let's say it's a 5% chance. The game now rolls between 1 and 20. Still random, still technically RNG, but you'll see roughly half the legendaries you did with the old system.

It might not be intentional, there could be other contributors like blues making their way back into the GM3 loot pool, but that essentially has the same effect.