Under the normal reduction rule, 11 would just become 1 + 1 = 2, and the calculation would move on. Numerology's master-number convention interrupts that — treating 11, 22, and 33 as significant enough on their own to be left unreduced, at least at the final step.
The reasoning traditions give is that these three numbers are "doubled" versions of 1, 4, and 6 respectively, and are said to carry an intensified version of that base number's meaning. That's a claim about symbolism, not a rule of mathematics — the arithmetic itself doesn't care whether 11 is "special."
