so how come someone with 34DD sometimes appears to have bigger boobs then someone with 36HH?
Fake boobs of one size will often appear significantly larger than natural boobs of the same size. Implants are engineered to have greater definition, sit higher on the chest, and be more symmetrical than real boobs. Add to that the fact that many women with implants are often significantly thinner than women of the same natural boob size, resulting in imbalanced body proportions.
That's really the only reason that I could imagine a DD looking larger than a HH. But also keep in mind that most pornstars lie about their measurements anyway, just as they do their weight and age. It's a point of vanity. And many are also slow to update biographical information, even on their own websites. So it's possible that your 34DD has had surgery and gone up several sizes in the time since she last updated, while your 36HH has been exaggerating all along.