Most numerology numbers require converting a name or combining several date components. The birth number skips most of that: it's just the day-of-month someone was born, reduced to a single digit if it's a two-digit day.
Anyone born on the 7th needs no reduction. Anyone born on the 16th sums 1+6 to reach 7. Anyone born on the 25th sums 2+5 to reach the same place. All three birthdays are treated as sharing the same birth number and, in the tradition's telling, something of the same underlying disposition.
