If the actual return on an investment is different from the expected return, then it is likely that:

		When investors estimated the expected return they correctly weighed all of information that they believed would bear on the investment.
		Interest rates remained unchanged after the expected return was computed.
		Some unanticipated information about the investment was revealed after the expected return was computed.
		Investors didn't use all relevant information that was available when computing the expected return.
		Even though the expected return was incorrect, the normal return was estimated accurately.