BSAS 2010 Q10 item no.6

" The firm’s real estate are valued every 24 months by an external appraiser since they don’t have the expertise internally to value this asset class" I answered this statement as correct by the guidlines answers says it is incorrect claiming that real estate assets must be valued at least annually" My understanding is that real estate assets should be valued at least annually but when it comes to external valuation by an appraiser it should be done at least every three years. Am I missing something here?