Appraised price looks strange

[Leon Lee]

Hi, BPers

I am in the middle of applying for HELOC on my primary home and the bank sent an appraiser to appraise the value of the house. What he/she did was selecting three comps and adjusted their values based on the differences between my property and these comps. After the adjustments, the appraiser came up with a number on how much my house worths for today's market value. My question is, why the appraised value of my house is LOWER than the lowest adjusted prices of the three comps. For example, the adjusted prices of the three comps are: $320K, $325K and $334K and the appraised price for my house is $315K.

What is happening? I am asking because in the past, appraisers made all kinds of errors in the reports for my rental purchases. But I also want to see whether there are possible reasons for she/he to derive that number that I am not aware of. 

Many thanks

Lee