That's all it is. They haven't changed anything so far as I can tell. I got Edo Jin everyday until yesterday. One guy I talked to didn't get any so far. Another guy got Edo Jin last night, so they are still there. It's just random. You can eventually finish them from 3v3 unless they change it sometime later. Seems like opening more packs at once tends to give you slightly better odds since it seems like it gives you a little of all of the ninjas. But it's hard to say without hard data to validate.