OMG I have defeated RNG!!!
First off the angel who told me to try a different time on my evolution I normally do around dinner 5-7pm this time I did 5:15 am I finally got C/DP on my Machamp on try 21!!!! People told me odds were 2.6% I have no idea if time change made any difference but my god I owe you a few beers either way. Good luck all may the RNG gods be in your favor.
Answers
Huge congrats on your C/DP.
I'm not sure if it's clear to you or not that the 2.6% was the probability associated with evolving 20 Machamps without at least 1 receiving the C/DP moveset.
You can think of it as the probability of your journey to a C/DP Machamp being what it turned out to be. (Your particular journey had a 2.6% chance of occurring before you started).
It is a probability associated with "rolling the dice" a set number of times (in your case 21 times). The probability of rolling an actual dice and getting a 6, is 1/6 or 16.7%. Which happens to be the same probability of evolving a C/DP Machamp (because there are 6 possible moveset outcomes). With any single roll/evolve, you will always have a 16.7% chance of rolling the 6 or evolving the C/DP Machamp. That probability never changes. What does change is the probability that you will get at least one 6 if you roll the dice a number of times. In your case that number was 20 times (21 now) and the chance of rolling a 6 in 20 rolls is 97.4%, and 2.6% of not rolling a 6 in those 20 rolls (which was what happened to you).
Feel free to ignore this^ hehe.