Why 72%?

3 is in the heart family (3, 6, 9) and 11 is in the ground family (2, 4, 8). 11 is a master number, so it is matched as its root, 2. Ground with heart starts at 70. The nudge comes from the roots: 3 + 2 = 5, which leaves 2 when divided by 3, so the nudge is +2. 70 + 2 = 72.

Out of the 78 possible pairings, 43 score higher than this one. Rarity: 11.1% of birthdays from 1950 to 2009 give 3 and 6.5% give 11, so about 1 in 69 random pairs of birthdays land on exactly this pair.

The rule is our own design, not a numerologist's chart, and numerology is not a science.