For others that may be looking for this information, since I know it’s a very old topic. The second row that you’re asking about is required so that ACF knows with to return. As you can see, the first row contains a serialized array. Without the second value ACF will return an array of attachment IDs, the second row tells acf that it’s a gallery field and to return an image arrays for each of those IDs.
All ACF fields are the same. The only fields that will work properly without the second entry in the database are fields that store simple text values.
