Insofar as they are immune, is it because of some sort of foresight and/or better engineering practices, or because the immune chips are simply cheaper, less so

Insofar as they are immune, is it because of some sort of foresight and/or better engineering practices, or because the immune chips are simply cheaper, less sophisticated designs? Hint: The fact that ARM ranges from "completely immune" to "affected by the less-common Meltdown" gives away the answer.