There is nothing wrong with how it is dropped. There is a small, equal % chance per battle for everyone to receive Z-tokens. How much is dropped, we don't know. What is the % chance of getting it per battle, we don't know. But it is there, and it is fixed.
There's something called a RNG (aka, Random Number Generator) or in math, probability, that determines how "lucky" we are to get a good "number". This works the same way for the GGBs, where the rarer boxes are, well, rarer to get. But some people are still able to get maybe 5 in a row. Some may need to open up 100 boxes or so before they get it. It is entirely real-life luck.
So the further answer your doubts and questions, but it this way simply.
You were very lucky to get most of it in the first 30 or so battles. But thereafter, your luck started to decline.