SharePoint Renderlistdataasstreamapi and count

Hey all,

I am looking at the API calls coming from my SharePoint list to see if I can replicate the calls in app and have run into something.

When calls are made from the list if there are <5000 items it calls the API using
https://mytenant.sharepoint.com//_api/web/GetListUsingPath(DecodedUrl=@a1)/RenderListDataAsStream?@a1=...

But once it goes over 5000 it calls 
http://localhost:42050/..../SharePoint/_api/web/GetListUsingPath(DecodedUrl=@a1)/RenderListDataAsStream?@a1=...

Anyone able to explain why it does this?
They both seem to have slightly different functionality.
The first will error if you attempt to add an aggregation to the CAML the second appears to be able to count the column - just curious as I was looking to use this with power automate to get a count of some filters.