An agent that searches does not know in advance what it will get back. A query that finds nothing still costs a query, an empty result and an answer, and the window keeps all three: nothing is thrown away between rounds. So the cost of searching is not the cost of the search that worked, it is the cost of every search that came before it as well. Watch what one successful retrieval does to a window that two failed ones had barely touched.
Paid members: the worksheet and its printable PDF are below ↓