If you are seeing folders you didn't expect in Lightroom, this is probably because you used the wrong import setting when you imported them with the wrong options under Destination.
Also many of the folders are not listed in the lightroom library at all, even though they are really there. How come lightroom sometimes shows a folder in my library, and sometimes doesn't?
Lightroom is not a file browser. It only shows you the folder it knows about, and the ones it knows about are the ones which have at least one imported photo in that folder.
Expanding the parent folder 2015 should list all the folders within it as you see on your external drive.
Would request you to share a screenshot of your Lightroom folders panel and the folder structure on your external drive outside Lightroom if there is a difference in the folder structure.
Is the folder structure within 2015 folder on your finder window or the main parent folder?
Seems like your import settings are not setup properly and that is why some of the folders end up being a part of 2015 folder instead of being a main folder.
You might have 2015 folder setup as the destination folder at the time of import which was causing that.