Momentarily baffled, I realized that, despite appearances, the old frame was actually not square, in fact it was a parallelogram. I'd measured the height and width and assumed it was square. The previous (experienced) carpenter who'd built the doors I was replacing had clearly noticed this, and simply allowed for the misalignment in his design. He built perfectly square-appearing doors that mounted to the not-square frame. I had to go back and rework mine considerably for them to fit without looking ridiculous. They're still there and holding up well, but I also still think of this lesson on a regular basis in my day to day life now.