When looking at the Gallery Card documentation, their is a specific layout for Twitter Card Galleries:
We’re expecting to see the images 2 by 2, as they appear in the documentation.
However, there appears to be a second view of the gallery that a user will see when viewing a single tweet:
If this is the expected functionality, the documentation should probably demonstrate this. If it’s not expected functionality, there should be a parameter or additional tag to allow users to ‘force’ the 2x2 view.