The female has an 80% chance to pass it down, while the male has 20%.
100 .8 = 80 (Female)
100 - 80 = 20
20 .2 = 4 (Male)
80 + 4 = 84%
Or, you can have done it with the male first:
100 .2 = 20 (Male)
100 - 20 = 80
80 .8 = 64 (Female)
20 + 64 = 84%
Either way, it's a solid 84% chance. The reason why you're getting about 4/5 non Hidden Ability Dratini is bad luck. If something says 1/4 chance, it doesn't necessarily mean out of 4 tries, I'll get 1. It simply means for that instant, there is a 25% chance of getting what I need.