Like the above said, it's random, but i to noticed the same phenomenon, I think it is due to 2 things.
1. You pay more attention to what you are getting from multiple DEs, and you have a more noticeable jump in available mats no matter what at the end of multiple DEs than the same amount over a longer period of time.
2. While it's true that what you get is "random" to a certain extent, due to the way numbers are generated, true randomness does not exist. Furthermore, depending on what method is used to generate said random numbers, it is entirely possible to get "streaks" of of similar occurrences. This is a documented problem with early Random Number Generators, where the same numbers would pop up repeatedly, and certain numbers where more likely than others. I'm sure that modern day RNGs are much more advance but once again it it a noted issue that RNGs aren't truely random.
I find this to be true across all of WoW, when it's hot it's hot, when it's not it's not. I've had to many experiences where I'll get 3-4 greens in a row and then a blue or some other crazy random thing drop to believe otherwise unless Blizz specifically explains otherwise. Especially after Blizz announced that everything a mob drops is decided at time of spawn.
My advise is to take advantage of a hot streak when you can, and back off if your not getting the results you want.
How's that for a loaded answer huh?