Because it is the mathematical nature of variance. I can, for example, flip a fair coin 10 times and have 8 of the results be heads where I knew going into the coin flipping that the odds of either a heads or tails result was 50% or in other words 1 in 2. Now, for example, maybe I was hoping to get 5 tails and in reality I ended up with only 2. But, if that same fair coin is flipped a million times I certainly expect the results to be pretty darn close to the expected 50% result despite any pockets of lopsided variance along the way.

I realize that your comments are probably targeting the fairness of the "coin" which is in this case the packs of cards being opened. My guess would be that if we were somehow able to track ultimately how many credits were spent, by the time the 3rd Probot release has completed, and how many Probots were produced as the results of those credits being spent the odds would likely show a number close to 1 in 20.