M-M with subset matching rule is matching transactions without matching all parameters

We have created a Many-Many match rule with subset enabled in single data source match type. In this rule, we are matching 3 parameters -

a. policy should match exactly

b. state code should match exactly

c. amount should match exactly.

But this rule is matching transactions where state code is blank in both debit and credit transactions.

Questions -

1. Why it is not considering state code while matching transactions? How it can match on blank state code?

2. It shows different behaviour than 1-1 match rule with same matching parameters. In 1-1 match rule, it is not matching transactions with blank state code on both sides. Why?