When I frame a box, it off about 0.025 of an inch

Hello. When i create a box and try to frame it, everything is correct but the back side. In other words the width is correct, and the line it shows next to me is correct but the back side is off about 0.250 of an inch. I took Jays advise and created a 1 inch squire and line cut it and it was prefect. any advise would be greatly appreciated. Thanks Dale