value returned by EXTRACT, date_part

I notice that although EXTRACT(field FROM source) returns a value of
type double-precision,

for some field types,  it apparently rounds the value to an integer,  
(even though the exact value is not integral),

whereas for others it returns the exact value including any fractional
part un-rounded. Examples can be seen in the book :

SELECT EXTRACT(DAY FROM INTERVAL '40 days 1 minute');
Result: 40

rounded down from 40.0.0006944444444444445

SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '2001-02-16
20:38:40.12-08');
Result: 982384720.12

un-rounded

This seems inconsistent to me - and undocumented as far as I can tell.

Is there any reason for this?    And can it be relied on in programs?

Cheers,   John