wierd behavior

Question
I have two outdoor male cats, one is a Siamese and the other is a Russian Blue.  Normally when another cat comes to the house, they either run or puff up.  Recently another Russian Blue has come up to the house.  Our Russian Blue tries to get close to the cat, and does not seem scared.  The Siamese just watches.  I was wondering why they might be behaving differently towards this cat.

Answer
Anglley,

It may simply be that your cats know this cat and have befriended it.

I am assuming they are neutered, otherwise, this may be a female in season which would entice healthy whole male cats?

I know of no other explanation.

Best regards... Norm.