parece que o problema é causado por group by como as
summedTags
são as ocorrências de 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;