Sorry English is not my main language can u explain it please I have the feeling it's a joke but I don't get it ... What's the difference between stain less and stain free ... Both are without stains right?..
The other two comments have it right. "Stainless" would generally be read as "free from stains", but the material really isn't free from stains, it just rusts slower than most other types of steel. It's sort of a pun that stainless steel merely "stains less", and isn't completely free from it.
I think it's meant as a joke ("less" as in "fewer").
In all seriousness though, stainless steel still gets rust, it's just much more resistant to it. From what I've read in comments, Tesla used a cheaper grade of stainless steel that is less resistant.