Wine theifs are real long... If the wort And the top off water weren't completely mixed, the wort sits a little below the top off water and using a wine thief, i think you are likely to pick up the sample from the higher gravity wort. My guess is, you hit close to the 1.071 in reality but it just didn't show up do to it not being completely mixed. When its done and drinkable, I'm sure you'll be able to tell if its 7% alcohol or 9%. So That should tell you.