Cross back top

I loved the look of this cross back top in Love Sewing magazine so a made it up in the floral viscose I picked up from Walthamstow. Its an odd one because it should fit perfectly but it slips arounds all over the shop. I do like the cross back shape though so might try this again in another fabric. The pattern asks you to face the arms, neck AND the backcross pieces. I'm not sure it was really necessary - the arms and cross pieces could have been hemmed instead and I still think it would have worked.