Rob,
Agree with Mamatha.
The Activities Associated View is a Special View, you can choose whether you want to only show the "this record only", or all related activities, and it is a rollup.
So let's say you have 12 records to the related Activities to this Account record, it means that all of those activities are linked to this Account DIRECTLY, connected by a REGARDING Field and some connected as Activity Party, for example Phone Call "Call To/From", eventhough your regarding is a Case, but if this Account is the Call To or From, it means it is related.
And for the All Activities view with Related "Regarding" record criteria, it is all of the activities related to this Account record, including child, grand child, and also Contact.
So for Example: Company BizNet, you are the Contact person.
Then, you make a phone call, it will also appear in the BizNet Activities, if you choose the "Related 'regarding' record", not the this record only.
So, let's say you have Case then create activity, it is also link to your company record, BizNet Account.
And also BizNet has branches or sub-accounts, then all of the related activities will also be rolled-up.
So you can see the whole activities related either directly or indirectly to the Account.
But, this is only in the Associated View, not available in the subgrid, in the subgrid you can only show Activities that linked to an Account using "Regarding" field only.
Hope this helps you, Rob.
Thanks.