One week is always 7 days. But one month (and year) is not always the same length. If you add or subtract n months from a timestamp or date at the beginning or the end of the month, it returns the beginning or end of the month n months away. Here's an example showcasing this using the fact that 2016 was a leap year:

    $ select '2016-03-31'::timestamp - '1 month'::interval;
    > 2016-02-29 00:00:00

    $ select '2016-03-31'::timestamp + '11 month'::interval;
    > 2017-02-28 00:00:00

    $ select '2016-02-29'::timestamp + '1 year'::interval;
    > 2017-02-28 00:00:00