Why Disk persistence size is almost double the size of BW system for HANA

For e.g. : Our current BW system is 3 TB, following calculation based on that :

QUE : Why we need 6+TB Disk for 3 TB BW system ?

RAM = (Source data footprint u2013 50GB) * 2 / 4 + 90 GB

= (3,000 GB u2013 50GB) * 2 / 4 + 90 GB

= 1,565 GB (= 25 Units : SAP Licenses HANA by 64 GB Units

DISK (Persistence) = 4 * RAM

= 4 * 1,565 GB

= 6,260 GB

DISK (Log files) = 1 * RAM

= 1 * 1,565 GB

= 1,565 GB