The "Instaled Apps" meter in your device shows the used space on your rootfs. In reality there are tons of other things than "apps" that might fill uo your rootfs... it depends.

There are lots of things you could do, you might check (by "du" for example) what's eating up the space and remove something.
Or you could repartition your device, as it is fairly silly anyway that a 64G device is partitioned as 4+2+58, I'd suggest something like 8+8+48 for example