Thanks!
Actually the problem with the 15mm is that the stock studs are too long. With the spacers in, the stock studs end up in the grooves on the back of the wheel (when the wheel is mounted) and with the 15mm spacers, the wheel essentially bottoms out and doesn't sit flush. With 20mm and 25mm spacers, the stock studs do not prodrude as much past the spacers surface therefore the wheel can sit flush on the spacer.