Your appraiser was a Cal Bears fan and theirs bled Cardinal Red?
But seriously, there are a lot of variables... a couple could be...
Was yours a drive-by and theirs a full appraisal, or vice versa? Did you both get the same appraiser? Was it appraised on the same day? If it was a different appraiser, did both appraisers have access to all of the same data? Are all the upgrades similar? Is the lot size similar? Did one appraiser give greater weight to certain comparables (such as distressed sales) than the other? Was one appraiser local and another not as familiar with the area? Was it for the same purpose such as financing, litigation, or tax/estate planning? Was the goal of each appraisal the same, such as determining the market value, assessed value, insurable value, use value, etc.? If financing was involved were your mortgage types the same/lender's instruction to the appraiser the same (for example one was wanting it to be compared to only rental properties and the other to all sales, or one wanted all sales and one wanted to exclude distressed sales)? Did the appraisers interpret the instruction the same?