The distinction is: If you take a printer, and replace part A, and a time later replace part B, and a time later replace part A again, you've thrown out three

The distinction is: If you take a printer, and replace part A, and a time later replace part B, and a time later replace part A again, you've thrown out three parts - 2 of A and 1 of B. If you take a printer, and A is broken so you replace the printer, and a time later A is broken again, so you replace the printer again, you've thrown out two entire printers. These are not the same.