Interval "1 month" is equals to interval "30 days" - WHY?

Hello.

I've just discovered a very strange thing:

SELECT '1 mon'::interval = '30 days'::interval --> TRUE???

This returns TRUE (also affected when I create an unique index using an
interval column). Why?

I know that Postgres stores monthes, days and seconds in interval values
separately. So how to make "=" to compare intervals "part-by-part" and not
treat "1 mon" as "30 days"?

P.S.
Reproduced at least in 8.4 and 9.1.