Hi Heidi,
Outside of a few operators that must run on Orchestrators, all other operators (including Ping) can run on an Orchestrator or an Agent. This is configured under the Execution Settings page for the operator.
While I am not sure why, note that the Ping operator uses the Echo protocol instead of the ICMP protocol. If you ping manually from your own workstation, chances are that it is ICMP so there is a difference between the two.
https://en.wikipedia.org/wiki/Echo_Protocol
Thanks,
Tom