Tbf, my last phone(Xperia Z5) did the same thing - it would show me that apps were taking ~22GB, but when you displayed them and actually added up the numbers it was about ~12GB. I actually went and uninstalled pretty much everything I could, and "apps" were still showing as using nearly 10GB. I only got it back after a full firmware reset.
On Android, you can install any random file manager and explore & clean the filesystem yourself (many apps litter on the virtual sdcard). There's also a DiskUsage app (https://play.google.com/store/apps/details?id=com.google.and...), that will scan the device and tell you, where are the files that take up the space and allows you to delete them.
