This happens to me all the time and I think it is quite simply fabric stretch, especially when you have bias cuts as in the triangles. Starching prior to cutting is supposed to help, but I don't like working with stiff fabric. My tutor told me only ever to PRESS with a dry iron. This helps, but it it is all down to the handling of the fabric and that takes a lot of patience and practice. I regard it as a bonus when I get a true square.