It's also because in the interest rate range we're normally dealing with (5%-10% APR) the approximation of 1+x/100 as 1.01^x in the derivation of the rule is of

It's also because in the interest rate range we're normally dealing with (5%-10% APR) the approximation of 1+x/100 as 1.01^x in the derivation of the rule is off a bit, and something like 1.0097^x works somewhat better. ln(2)/ln(1.0097) is about 71.8, thus the use of 72 and it's convenient divisors.