question about coherent_access in presentation qos

Hello, I was working with the rti-dds examples about the 'presentation' qos, when i set the publisher's and subscriber's presentation qos as access_scope = INSTANCE_PRESENTATION_QOS, coherent_access = true. And I published some data between begin_coherent_changes and end_coherent_changes, it supposed behave as the same as the coherent_access = false since the description about the access_scope = INSTANCE_PRESENTATION_QOS in DDS specification is like this "If access_scopeis set to INSTANCE, the use of begin_coherent_change and end_coherent_change has no effect on how the subscriber can access the data because with the scope limited to each instance, changes to separate instances are considered independent and thus cannot be grouped by a coherent change." But the result I get was the same as the access_scope = TOPIC_PRESENTATION_QOS, that is the samples grouped into a coherent set, the datareader read the samples when the publisher end_coherent_changes. I think this is not correct according my understand. Can anyone explains to me? Thank you!