Custom bullets added from files on your computer are embedded in the style sheet. Even if they are not used, they remain embedded in the style sheet until you delete them from the style sheet.
Package bullets only remain in the style sheet if they are used in a style. While you can manually delete them from the style sheet, it is not necessary. If none of the styles in the style sheet use a package bullet, the Style Editor removes the package bullet from the style sheet the next time you open it. The image is not deleted from the package.