Sub panel neutral and ground not bonded so why continuity between them?
The feed for your sub panel comes from your main panel the grounded conductor neutral and grounding conductor equipment ground are connected in the main panel so measuring continuity in a sub is normal even when the grounded conductor is isolated from the box and the grounding conductor.
This is normal the resistance value is usually quite low just a couple of ohms or close to zero. The length of wires and any resistance at the connections is all there is.