Not a rounding thing.

As best as I can tell it is probably a cost thing.  Nearly every vehicle that rolls off an assembly line these days has all metric hardware, especially wheel lugs / studs.   It may be cheaper for the spacer manufacturer to use metric studs and nuts than standard, especially overseas.

Close, but no cigar. Get the correct lug nuts.

Alessandro wrote:

This is very interesting because i have to deal with it  every time i lose a nut.
The us definition of bolt size refers 1/2 - 20   as    1/2 inch  20 threads 
but here in europe  bolt size refers  12 mm x 1.25      as  12 x  pitch to pitch distance
The diameter is quite the same but i can't say nothing on threads .
could be very very close but not exactly the same thing.
If you make a measure from pitch to pitch from your 1/2 - 20  and change it in millimeter you should have something close to 1.25 as result  . if is like that you have win !
Tomorrow i can try to buy a 12 x 1.25 and try on my lug  . Maybe i got surprise  and fits 
ciao !

IMHO you should never ever mix metric and SAE. In this specific case it might be possible to put a 1/2 - 20 nut on a M12x1.25, but as "20" in "1/2 - 20" mean 20 threads per inch or 25.4 mm / 20 threads = 1.27 mm/thread it's not correct and as 1/2 inch = 12.7 mm and M12 obviously is 12 mm you might end up with a bolt/nut combination which is half as strong or worse as it should be.