The term “net asset value,” or NAV, is so commonplace for anyone buying a mutual fund, investors hardly give it a second thought. It has a standard definition. The number is audited annually. There is rarely a reason to question it.

But in a corner of the market that has grown rapidly—real-estate funds—the NAV label, while ubiquitous, doesn’t have the same meaning. In most cases the funds, known as nontraded real-estate-investment trusts, have broad leeway to define their NAV measurements however they want.