volume capacity advertised has changed after formatting

Hello Gurus,

When I present a 4GB volume (it's an iSCSI LUN) to a VMware cluster, after formatting the volume, the volume capacity has changed from 4GB to 3.75GB and it also shows the free space that can be used out of the 3.75GB which is 3.10GB. In Windows OS when the 4GB volume (iSCSI LUN) is assigned, after formatting it as NTFS, it would still shows as 4GB capacity once you right click on that volume in Windows explorer but not all of that space is usable which I understand.

What's the explanation behind why the capacity advertised in VMware has changed from 4GB to 3.75GB after formatting volume? 

Would greatly appreciate your response on this query.

Thanks,

Alyana