That’s correct. All topics have a first post as part of the topic creation. One couldn’t very well create a topic with only a topic header. That first post isn’t counted as a post (reply) in a post query (filter-5).
When you query for posts, that’s a query for new Replies to Existing Topics - excluding the OP. That is the expected behavior - to show only the new posts.
To include Topics (created by that user) as part of the query, then Filter=4 should return the topics that user created (including the OPs as part of the count [3]), and their replies [4]. That would return the 7 you see in the report. 3 Topics created + 4 replies to give a count of 7 (3 OPs + 4 Replies).
Anyone else more knowledgeable please feel free to jump in.