How to find data for REST API Activities GET

Our instance is located on sugarondemand. 

According to the schema, we can see "comments" and "activities" relation.

I tried to use Postman to test these REST API.

a. Run the API "/Comments GET" and it returns parent_id. This is the activities id. 

b. Run the API "/Activities GET" and use offset to find the above activities id.  But I cannot find it.

c. Run the API "/Activities/<id> GET" and it returns the results.

Question:

Why I cannot get the result by procedure b. (using offset) ? 

Reference:

1. schema

2. api doc

Thanks,

Calvin