iOS High Data Usage

Data usage in iOS combines the actual app's data usage along with the VPN adapter usage which results in showing 200% of the actual data usage.

Note: Zscaler App uses the VPN adapter to receive the traffic from the Apps and then forward it out to Zscaler ZEN nodes accordingly.

This behavior is seen by other Apple users as well and they have raised a Bug with Apple. You can find more details in the below link