Replacing lightbox image in existing lightbox

So I’m just curious: if I have an existing lightbox and decide to swap out one of the images, Blocs changes the status on the right to “alternative image”. Everything works fine, the new image is shown both as thumbnail and in the lightbox.

I can go back and change the setting from alternative image to “current image”, but since everything works fine without changing this setting, I’m just curious why this is … ???