IIRC a retrospective by one of the designers listed the default promotions as a careless mistake, because the original—offhand—reasoning was that the registers

IIRC a retrospective by one of the designers listed the default promotions as a careless mistake, because the original—offhand—reasoning was that the registers are 16-bit anyway, so they might as well widen everything when it’s free (on the only machine the language was implemented on at that moment).