Where is the accuracy?

I followed all the "Tips" and I have yet to get an "accurate" measurement. Every measurement is off by 5 to 7%. Is that considered accurate? I was really hoping to use this for scaling photos of larger objects. If Im doing something wrong I hope someone can help. I gave this a 2-star for the idea and the use of measuring small objects. To me it needs real improvement. Id ask for my money back but that is not an option at the AppStore.

chainstrainer about RulerPhone - Photo Measuring