The reason is that the hub collar (marked red at the picture) is so long on our cars and the 10mm spacer doesn't have enough room to make it deep enough for the hub collar. So spacer won't sit tightly against the wheel hub. Is there like 0.5-1mm cap??
12mm fits because it has the extra 2mm.
There is few solutions to this. I know people who have grind the ~1mm out of hub collar. Also i heard some 10mm spacers don't have that problem, but don't know what brands.
But why don't you just buy 12mm spacers it will solve your problem and i doubt you will notice 2mm difference.