While this is somewhat inelegant, it is a result of the fact that countries A and B (for any given reason) did not choose to include each other in the group of countries with which they would agree to apply the tie-breaker rule.
虽然这种情况令人难堪，但其原因是，A国和B国(无论出于何种原因)均不把对方归入其同意适用防止双重居住地征税规则的国家集团。