Sashing can be a problem. Fabric is not a stable medium and can stretch or shrink no matter how carefully we handle it. I was taught to measure all sides of the piece and then use an average of all of the measurements. Pin the sashing at the middle and the two ends and then ease the rest together. Always remember to "press" rather than iron. Fabric especially under steam, stretches and or shrinks.
From the pictures it looks like you did an excellant job.