I would first find out if the appraiser made a typo before you take action to back out. I have seen it happen where they (appraiser) had accidentally left the prior appraisals numbers in their system. I would also find out when your contingency period for inspections is over & make sure you are comfortable with the whole process before you decide to move forward. Act quickly & don't delay.
I had this happen once with a condo I was buying. It was a condo in a two family house, and it turned out that the seller was using the foundation measurements for their determination of square footage. You should talk to your agent about your options. I eventually backed out of the transaction.