The attachment is by blood vessels kind of embedding into the wall and the blood kind of percolates from mum into the umbilical cord. And the movement is kind of relative. Imagine a little stretchy wart on a balloon thats not blown up. The wart grows and the ballon blows up, so it might have looked close to the mouth piece when the balloon was small but it grew towards the top and the balloon stretched so in the end it was only the edge near the mouth piece and not nearly as close because the bit nearer the mouth piece also sretched as the ballon blew up....OMG does that make any sense?

Having the placenta near the scar doesn't = bad every time either. There are some complications SOMETIMES, nOt all the time, where the placenta kind of gets stuck to the scar, but the large majority of time it doesn't, and they will be able to tell if it is and then it won't be such a big problem....

Hope all that made sense
I am sure your placenta is lovely and healthy and wonderful!