Gallery problem

In an exisitng post I installed a gallery via “Add media” with 2 photos from the media library. When I look at the result there are 3 photos!!
I selected nos 2 and 3 in the library, but no. 1 is appearing also.
When I select nos. 4 and 5, nos 1,2 and 3 appear.
On a new post the gallery does not appear at all, although it shows in Visual edit mode.
I have no gallery plugins installed.