Oracle Pl/sql No Type Checking Inside The Declaration Section

I always have wondered why Oracle doesnt checks for the type casting inside the declaration section of a pl/sql block and it also ignores the checking of size mismatch inside the declaration section. When you do one of the above mistake in side pl/sql stored procedure it compiles the code successfully but it gives the runtime error. Ideally it should go inside the exception block but it doesnt .Can Any one tell me why. I didnt find any proper ans. on any forum till now