Sqlite comparison of the same operand types behaves differently

Based on Sqlite docs: https://www.sqlite.org/datatype3.html#type_conversions_prior_to_comparison , especially this statement: > If one operand has INTEGER, REAL or NUMERIC affinity and the other operand has TEXT or BLOB or no affinity then NUMERIC affinity is applied to other operand. I would expect the following query:

CREATE TABLE invoice (
  id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
  amount DOUBLE PRECISION DEFAULT NULL
);

insert into invoice (amount) values (4.0);
insert into invoice (amount) values (15.0);
insert into invoice (amount) values (4.0);

select *,
    typeof(amount), amount = '4',
    typeof(sum(amount)), sum(amount) = '4', sum(amount) = '4.0', sum(amount) = 4
from invoice
group by id;

to return the same result for sum(amount) = '4' as for amount = '4' for each row as both operand types have the same type in each comparison (verified using typeof(), for non-SUM() the comparison is working as expected). Demo: http://sqlfiddle.com/#!5/59238/2