This is a known bug which only affects the user interface. Version 5.1 (and older) of the Delphix Engine user interface only reports the number of virtual CPUs and doesn't take into consideration the number of "cores" of each (virtual) processor.
The defect only affects the value displayed in the user interface and not the internal behavior of the Delphix Engine. It will be corrected with the next major release.
You can verify with the CLI (or API) that the system is correctly configured by connecting as a "sysadmin" user:
ssh sysadmin@<delphix-enging-hostname> system get processors