RAID 10 is supported on SPARC T5-series servers with four or more disks. For an even number of target disks (four or more) RAID 10 is the functional equivalent of RAID 1E on an odd number of target disks (three or more).
With the FCode-based RAID utility, a RAID 10 volume is created when when you enter this create-raid1e-volume command that includes four target disks:
ok 9 a b c create-raid1e-volume
This is expected behavior.
Refer to the SPARC and Netra SPARC T5 Series Servers Administration Guide for more about creating RAID volumes.