Imagine a database trying to solve

    SELECT client_id, invoice_id, invoice_status, invoice_date
    FROM invoices
    WHERE client_id IN (123, 567) AND invoice_status != 'PAID';

Imagine that there is an index for client_id.

The DB engine can scan the index, finding all the pages in the table that contain rows with one of those two client_ids in.

It requests these pages as soon as it finds them in the index.

It scans the pages in the order it finds them completed, though, which can be a completely different order than it requested them in.