The general theory is that they used a cheaper grade of stainless, specifically one that is still magnetic because it makes material handling easier during manufacture, meaning higher iron content, meaning more prone to rust if you don’t pay extra for the clear vinyl wrap. People will say the “real” name of the material is “stain less” steel, which is not true – “stainless steel” is just 1910’s marketing wank – but it is accurate enough as a description.