Why some system functions in T-SQL without parameters have parenthesis while others don't?

Question

I never remember if a system T-SQL function, with no parameters, requires parenthesis, so I wonder if there is a reason.

For example: CURRENT_TIMESTAMP is a function that doesn't require parenthesis, while SCOPE_IDENTITY() requires them.

Answer 1

I think the differences are due to whether the function is a part of the ansi sql standard (which current_timestamp is) or whether it is a sql server function.

Answer 2

I would rather think CURRENT_TIMESTAMP AS A SERVER VARIABLE..