If you want to be able to mow up to the edge of the lawn you either need to leave a gap or have the edging stones slightly lower than the level of the lawn.

Hi, welcome to GoY, I agree with Mg, the gap is there to enable you to cut the grass right to the edge, not to allow the lawn to expand, although it probably will, especially if it gets walked on a lot, Derek.

That's true Derek!

THANKS :)