Recently lenders are requiring their appraisers to use comparable solds no older than 3 months.
Many of homes the real estate markets across the nation have depreciated in values this past year or two.
On the 1st appraisal, lets say done spring of '07, theoretically the appraiser could have used comparable sold properties as far back as spring of '06.
Currently, lets say an appraisal done December '07, the lender could have required the appraiser to use comparable solds no older than September '07.
If the market of that home has declined in value since spring of '06, the result would be a lower appraisal value for that proeprty, given prarameter requirements to establish current value.
Best of luck