Small sample size.

You could flip a coin 20 times and get heads 15 times and it wouldn't be abnormal or a roll a die 20 times and get a six to show up 10 times and it wouldn't be abnormal.

Anecdotal: I made a gryphonskin ring a while ago. I had a 22ish% chance to meld vitality II on there. I went 1/20. It was pretty bad luck. Very bad luck.

Next day I made a Gryphonskin wristband. I successfully melded the 40% chance, 22% chance 18% and 11% (or whatever the numbers were). But I went 4/4 on overmelds. That was also against the odds.

I don't doubt the RNG. But just like any RNG, it can land on "bad" seemingly often in a small sample size.