That's true. It's impossible for any appraisal to be 100% accurate. If you are really serious about getting a valuable domain appraised accurately, you might want to try getting several professional appraisals, then average out the reasonable (mid to low level) ones, and cut that number in half (since appraisals are nearly always overly optimistic). In most cases, doing that will get you a somewhat accurate (but by no means foolproof) value.