Hola Girone, en este link te lo explican muy bien
Básicamente: "Other is contacts, calendar events, notes, email stored locally, as well as data created and stored on the iPad by apps. The system software (iOS) also requires a small part of the available storage capacity. The general range for most folks is between 1% and 5% of your capacity. "
Vamos, que si quieres minimizar eso, conseguirás algo si eliminas la cuenta de correo del ipad. Lo demás casi que no está en tus manos.