(though it's not clearly named, it's not the attribute value, it's the attribute friendly name)
It is the attribute value within the returned JSON. The PAV is like the lookup key of the JSON object, and exists for other Activity Type IDs as well. It is not meant to refer to the "value" as in the "Marketo custom field value".
BTW there's also a field called primaryAttributeValueId returned, which is numeric, but no idea what that is. Am I missing something?
I think you are, because the primaryAttributeValueId for Activity Type ID 13 is the id of the field returned from the Describe Lead call.