I suspect one reason why is because the brokers are providing an opinion of the price based on its as-is condition which includes its transactional (REO) condition vs. appraisers who, sometimes, exclude that transactional condition and appraise the property as if there were no REO stigma (if it affects value) attached.
While the work-around (for those who believe the REO stigma cannot be considered in an as-is market value) is to order two values based on two different value definitions within the appraisal, in your example, why do it when you can obtain a substitute that works as well?
The specifics of REO assignments is a discussion (which I got sidetracked above) is for another thread!

The example of a substitute for an appraisal is the relevant discussion here.