The leading auto insurer's stock looks cheaper than it has in a while, but the reasons behind the discount warrant a closer examination.Rarely has Progressive (PGR +0.85%) stock looked cheap. It is one of the best-run auto insurers in the U.S., with shares usually trading at a premium compared to most insurers. But after sliding about 21% from their all-time highs, shares finally resemble a bargain -- at least on the surface.So, is this pullback a rare chance to buy a high-quality insurer at a more reasonab ...