What you have done is cut “half square triangles” – so the long side is cut on the diagonal (bias). As a result, the ‘long edge’ of the finished unit (the flying geese unit) is now on the bias.
The instructions given were for “quarter square triangles” so the long side is cut on the straight grain. (You are getting 4 triangles from the two cuts of the original square). This is because you need to have the straight grain along the major seam lines.