While you can specify the number of stripes per object, the specification you make in that policy option is a minimum requirement. There are a number of reasons why VSAN may determine that it needs additional stripes over and above what you have specified by policy. Large objects need to be striped by virtue of the VSAN architecture. Additionally, no VSAN component can span more than one physical disk. If the overall levels of free space on any given disk are small, or if the physical disks are of a very low capacity, VSAN may need to implement additional stripes in order to build the object.
Finding that an object has more stripes in a mirror than you have specified by policy is not a cause for alarm and is completely to be expected.
The key difference between the stripes you specify by policy and the stripes implemented by VSAN for internal purposes relates to rules surrounding how those stripes can be placed. When you specify some minimum number of stripes by policy, those stripes must reside on separate physical disks. If VSAN determines that it needs more than your specified number of stripes, it can place multiple stripes on the same physical disk, as long as any failure-tolerance and explicit striping rules are not violated.