I Had My Eyes Checked Today, And The Results For

I had my eyes checked today, and the results for my eye exam changed after my eyes were dilated. My regular perscription is -1.00 for my left and -0.50 for my right. When i had the exam the machiene read that my left eye was now -6.25 so the doctor dilated my eyes and did another check with the machiene and it said that my left eye was now +0.75! How is this possible?