Probably the software. Unlike the newer tablets/phones that has storage analyzer, my tablet is pretty outdated and only shows a few data such as apps, videos, documents, images, and audio. It doesn't show system/ software storage.
Yup, Bern2Hell's right, you should probably uninstall some of the apps you're not using. Remember that you're working with 32GB internal storage, so it's going to be a challenge fitting everything you need in a single tablet.
On another note, I know the Galaxy Tab S3 supports MicroSD cards up to 256GB -- maybe you could get one and use it for file storage?
