Grid ppi not accurate to canvas
I have been trying to set a grid for my image but the grid is constantly a half inch or so too wide or small. My image is 12" x 9" and says it's at 72 ppi, which should mean when I set the grid to 72 pixels I should get perfect one inch squares, but when I turn on the grid and set it to 72 it's not right? I don't know if maybe I need to change the pixels per inch or the pixels the grid is set too or if I've measured wrong but I would appreciate help. 
