Hi there!
The bandwidth showing in our storage device manager must be accurate.
This point is very clear. About your problem, please check below.
Your OS is in VMware, right?
Storage receives write bandwidth = copy speed shown in windows + MBZERO bandwidth.
The MBZERO bandwidth is due to the thick provision lazy zeroed.
When the space writes the first time, VMware will send zero bandwidth to the storage, as said in the VMware document.