My observations tend to agree with Art's. I use double tied-on nock sets and usually the gap will widen after a few shots from the arrow nock slightly prying them apart due to the string angle at full draw. I strongly feel that a double nock is advantageous, but the gap in the pic doesn't look all that excessive. There have been pics posted with much worse slippage. It probably is affected by nock tightness also, but too tight nocks are just as bad. I tend to over draw an arrow a couple times, just to loosen up the muscles before a shooting session. I consider my nock fit to be near perfect, but if I let down after a full draw, the arrow will not be completely "seated" and the string will be more pinched out between the nock "ears". Don't know that this hurts anything as long as it doesn't cause dry fires.