SFH and Rental Property Disconnect

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I just read Marc Stephan's "Unlimited Real Estate Porfit," and loved it. Here in Tucson, however, there seems to be a disconnect between the single family housing and rental markets. Prices are up by 100% in the last few years for homes, (indicating the expansionary phase by his theory), and yet apartment rents have gone nowhere, and there are still empty apartments everywhere. It's an interesting situation. Has anyone seen this in other places?