The MySQL query with SUM () does not return the expected results

The query:

SELECT
    id_data,
    id_tag IN (75) AS tag1,
    id_tag IN (12) AS tag2,
    SUM(id_tag IN (75, 12)) summedTags
FROM
    tags_inservice
WHERE id_service = 1
GROUP BY id_data
ORDER BY summedTags DESC

The result:

id_data    tag1    tag2  summedTags
-------  ------  ------  ----------
   3109       0       1           2
   1956       0       0           2
   1928       0       0           1
   2738       1       0           1

What I think is wrong:

The summedTags rarely matches the actual sum of the tags! In the example id_data3109 has 0 + 1 = 2 which is wrong, and id_data 1956 shows 0 + 0 = 2 which is wrong too, etc, etc. What am I doing wrong?

By the way, I also tried adding HAVING SUM(id_tag IN (75, 12)) = 2 with similar results.

Some more extra info if you need it:

I have a table tags_inservice that contains a bunch of data (represented by id_data, an id from another table) and the data can be tagged by different tags. An id_data can have zero or more id_tags so there could be more than one row with the same id_data value. In fact, sometimes it's valid to have the same id_data tagged with the same id_tag more than once.

DESCRIBE tags_inservice;

Field       Type     Null    Key     Default  Extra
----------  -------  ------  ------  -------  --------------
id_intag    int(11)  NO      PRI     (NULL)   auto_increment
id_tag      int(11)  YES     MUL     (NULL)
id_service  int(11)  YES     MUL     (NULL)
data_type   int(11)  YES             (NULL)
id_data     int(11)  YES     MUL     (NULL)

---

seems the problem is caused by group by as the summedTags is the occurrences of id_data

SELECT
id_data,
sum(if (id_tag=75,1,0)) as tag1,
sum(if (id_tag=12,1,0)) as tag2,
sum(if (id_tag in(12,75),1,0)) as summedTags
FROM tags_inservice
WHERE id_service = 1
GROUP BY id_data
ORDER BY summedTags DESC;