VPlex Mobility Jobs Transfer-Speed?? Help??

I'm using the vplex to migrate data from one array to another and typically set it to 1M as the transfer size.

My question is, and i'm hoping someone out there could explain this to me, at 1M im seeing the reads and write on the target array as 65MBs... If i change it to 512K the reads and writes on the array drop to 13MBs.

I'm expecting 1M to transfer at 1M or is there something else i'm not accounting for?