Its RNG.

The highest base HQ chance you get is 15%. As you noticed, Unearth 2 and Leaf Turn 2 (level 35, 300 GP) only add 30% to that, for 45%. Mind of the Mountaineer (level 78, 300 GP) adds another 20%. That, for 600 GP, is an additional 50% on top of base, for 65%. Dredge and Dredge 2 add 10 and 20% respectively, for another 100 or 200 GP respectively, but only for one swing. So you can get it up to 85% if you have the 800 GP to spare.

Unearth 3 (level 63, 300 GP) adds a random amount, up to 100%. You might get lucky here, might not. Its range makes it better than Unearth 2 but there’s no telling if it will be a lot better or merely equal.

The RNG, however, comes from the node itself. Nodes usually spawn with some special effect. At times, that special effect is a bonus HQ chance for every swing, unlocked by having enough Gathering, enough Perception, or enough GP.

All that aside ... why do you need HQ? Assuming decent gear and not being too far below the recipe level, you should be HQ’ing stuff you craft from entirely NQ mats. This increases the exp/craft you get crafting, and reduces the amount of space in your bags you need for materials, unless all you stock are the HQ materials anyway.