Explicitly granting permissions to update the sequence for a serial column necessary?

Recently I did create a table as a superuser including a serial id column, e.g.,

create table my_table
(
    id serial primary key,
    data integer
);

As I wanted my non-superuser user to have write access to that table, I granted it permissions:

grant select, update, insert, delete on table my_table to writer;

At a random point in time after doing so, the insertions made by that user started to fail because the user lacked permission to modify the sequence my_table_id_seq associated to the serial column. Unfortunately I can’t reproduce that on my current database.

I worked around this by giving the user the required permission, like this:

grant all on table my_table_id_seq to writer;

Can someone help me understand

• why, at some point, the previously sufficient permissions might start to fail?
• what is the proper way to grant write permission for a table with a serial column?