Rewritten rows on unchanged values

I noticed how rows were re-written to a different location (new ctid) even
without changes to the values. This illustrate what I mean:

    CREATE TABLE demo (id serial, value text);

    -- generate a few pages of dummy data
    INSERT INTO demo (value)
    SELECT md5(s.a::text)
    FROM generate_series(1, 1000) AS s(a);

    -- ctid = (0,1)
    SELECT id, xmin, ctid, value
    FROM demo
    WHERE id = 1;

    UPDATE demo
    SET value = value
    WHERE id = 1;

    -- ctid = (8,41)
    SELECT id, xmin, ctid, value
    FROM demo
    WHERE id = 1;

I'm curious as to what would prevent keeping the row where it is and maybe
change xmin in place?

Thanks,
-b