Code4Fun

Hi there,
I have a question for those who really know how it works.

As far as I know, any high-risk merchant account has its own category (four-digit number). For example - the majority of merchants opened for porn have 5967 category (if I made a mistake ? correct me please).
Anyways, there is an opinion that acceptance of credit cards depends on a merchant account category.

For example - a transaction from the same credit card can be approved with 7273 merchant account category (dating) but declined with 5967 merchant category.
The reason is that a customer bank (the bank that issued a card) can block any transactions from merchant accounts having potentially dangerous category according to the bank?s opinion (the transaction is declined with a reason code 5 ? do not honor).
Can it be true?

And one more question.
I know cases from my own experience when a transaction was declined with direct merchant (same reason code - do not honor) but approved with 3rd party billing such as CCBill, Epoch, etc (transactions from Visa or MasterCard).
How can this happen?

Thank you