What is the status of your Storage Provider registration for that cluster? That might be the problem. -Jon
This is perfectly normal under a number of situations, and does not constitute a bug. For example, we will do this any time a VMDK is larger than 255GB. We will also do this when disk space is low. See Cormac's blog series Virtual SAN (VSAN) - CormacHogan.com and specifically Part 23: http://cormachogan.com/2014/04/23/vsan-part-23-why-is-my-storage-object-striped/
I also want to clarify that unlike what you said a RAID-0 does NOT consume additional disk space. It is simply a means to split a mirror into smaller chunks. In sum the size doesn't change though.
As for compliance, if you check out the definition, StripeWidth is a _minimum_ number of spindles per mirror to use. VSAN is free to use more spindles. Also note in the above cases where we use RAID-0 we may place two stripes on the same disk, as long as satisfy the StripeWidth minimum of spindles the user asked for.