Why remote.php (WebDAV) for first picture in picture-folder

When i look in a folder all small pictures are preview pictures loaded with “index.php/core/preview”.

Unbenannt

Bu when i now klick on the first picture it is loaded with “remote.php” (WebDAV):

Unbenannt

All other preview pictures in this folder do not use “remote.php”. Also all download links for the pictures do not need “remote.php”. Perhaps someone can explain it. Thanks.
I think it makes no sense.
Or it is nessessary to authenticate the user for using a different folder with WebDAV? But why the mini preview pictures does not need them but the normal pictures in the gallery need them?