In the MSA, the "Unused" means "disk space you could use to create further volumes or extend existing ones". In the server, "Unused" most likely means "disk space that is ready to accept files and directories, but currently doesn't contain any."
When you create a volume in MSA and present it to a server, all of that space will be "used" as far as the MSA is concerned: it cannot be used for any other purpose. MSA won't care about partitions or their contents: even an unpartitioned and unformatted volume will be considered "used" since that disk space cannot be assigned to any other server without risking a conflict and data corruption.
(Failover clusters, multi-node databases and clustered filesystems especially designed for multi-server access may allow exceptions to this rule of thumb, but they usually demand some form of cooperation and communication between the accessing servers.)
Note that for the MSA, the primary identifier of the volume is the WWN; the name vol_DATA_I_Burton is just an extra label for convenience of humans.
The server, on the other hand, will (usually) set up a partition table and a filesystem within the volume, and then track how much space is used within that filesystem by the stored files, directories and filesystem metadata, and how much space is still available for more files and directories.
When talking about used and unused space within a thing that has a Windows-style drive letter as its identifier (I:), you're almost always talking about the space used by/available for files and directories, not the raw unformatted size of the storage volume.
So I would expect MSA's "Used" value to match the server's "ALL" value (accepting a reasonable rounding error) - and that seems to be the case here.
