Do you have a 16GB device?
Have you tried to restart your phone?
Photos in photo library stored on device will occupy a small amount of space for thumbnail. That part can’t be deleted unless the photo itself is gone.
Some apps probably have left corrupted documents and data that could not be removed during normal process, taking up spaces that could not be reclaimed by removing that app. The only way to get rid of those are through erasing contents and settings.