Yes, that page is very simple, and very wrong. On my mom's iPad and iPhone, the app numbers on the storage page simply don't add up. Many gigabytes of space are unaccounted for, yet unusable. As far as I can tell the only way to recover the space is a factory reset, and it's only a temporary fix.
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.
