Why might one project have the highest pw while a different project has the largest irr? explain?

The difference may lie in the project life, sometimes referenced as the planning horizon. A project may have a large internal rate of return (irr), but a very short project life. A second project might have a lower irr, but a much longer life. In that case, the second project will return less per year, but will provide a return for many more years, resulting in a higher pw.