Why 76%?

5 is in the mind family (1, 5, 7) and 33 is in the heart family (3, 6, 9). 33 is a master number, so it is matched as its root, 6. Mind with heart starts at 74. The nudge comes from the roots: 5 + 6 = 11, which leaves 2 when divided by 3, so the nudge is +2. 74 + 2 = 76.

Out of the 78 possible pairings, 21 score higher than this one. Rarity: 11.1% of birthdays from 1950 to 2009 give 5 and 0.9% give 33, so about 1 in 516 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.