Because Damian is Bruce's One Real Son. Apparently.
I know! WTF on Dick referring to Bruce as 'Your Dad' to Damian? That's bizarre.
one son doesn't usually refer to a shared parent as "your dad" to another
Agreed. It's very unnatural and doesn't sound like Dick at all. And yes, it DOES sound like this comic's trying to remind me that Dick doesn't count as Bruce's son. Also, that Damian does.
It's... very off. IMO.
We've already been reminded by Alfred at least twice and several times by Damian. (I'm going to have to go back and count the reader reminders.) Now, third issue, it's Dick's turn.