It's what nature intended. The ewe anywhere is sexually active from Autumn to Winter, and she is designed to produce young when they can be fed, when there is grass growing. Here that means that we have one natural lambing season. Her cycle is 17 days or so. Sheep farmers have the ewes mated so that, as said, they don't get an enormous crop of lambs in the same week; this could happen naturally, otherwise, because females kept together develop a synchronicity of oestrus (happens with our species, too!)