AllWemotec rotors are balanced the same, even the little 50mm is balanced now as I went to do one last week and found green epoxy inside it. Lander "may" be statically balancing their rotors on the big fans before they fit them, their rotors are better molded than Haoye so they actually are centered on the bore, that in itself makes it easier to balance them. Of course they could still be using the harmonic method, only way to find out is strip it and look, bet you dont want to do that just for curiosity's sake!
The 70 and 90 also run very smooth when assembled from the factory, when you have a parts bin with 1000 rotors etc to choose from its cheapest to just swap things back and forth, time is not such a big cost inChina. After I ordered a pile of spare parts from them in 2008 and got a box of unusable rotors (looked like all the rejects they couldnt get to run smooth) I asked specifically how they were assembling and checking the fans, this is how I know. Whenever a Lander fan breaks we just fit Wemotec rotor and adapter to it, ends up being a better performer to boot!