Thumbnails in media gallery disappeared after deleting other image sizes

I have a photography blog where I upload my images sized to exactly how I want them displayed so I recently disabled the creation of different sizes when uploading. Since I wasn’t using the other images created prior to the change, I decided to delete them off my server to save space but when I did so the thumbnails in the media settings in WordPress disappeared.

So does that mean WordPress uses one of those images created to display the thumbnails? I was able to use the Regenerate Images Plugin to bring them back and it fixed the problem but how come it doesn’t happen to the newly uploaded images that doesn’t have other sizes created?

Thanks.