understand the pg locks in in an simple case

I have troubles to understand the pg lock in the following simple situation. 

Session 1:

begin;  update t set a = 1 where a = 10;

Session 2:

begin; update t set a = 2 where a = 10;

They update the same row and session 2 is blocked by session 1 without surprise. 

The pretty straight implementation is:

Session 1 lock the the tuple (ExclusiveLock) mode. 

when session 2 lock it in exclusive mode,  it is blocked. 

But when I check the pg_locks: session 1.  I can see no tuple lock there,  when I check the session 2,   I can see a tuple(ExclusiveLock) is granted,  but it is waiting for a transactionid. 

since every tuple has txn information,  so it is not hard to implement it this way.  but is there any benefits over the the straight way?   with the current implementation, what is the point of tuple(ExclusiveLock) for session 2?