Why no focus issues with D5100 or D3100 compared with D7000

I got my D7000 3 months ago, upgraded from a D5000 which I have been delighted with (and have kept).  I am not an expert at all but the D7000 is a jump up in understanding - I can tell that.  I also knew that before buying - looked at threads and did research before I committed to purchase.  I'm still learning the best auto focus settings and there is a bit of trial and error. That having been said I've got some real good pictures that I've been very happy with (mainly using Nikon 16-85 or Sigma 70-200 2.8).  The D7000 also feels great in my hands and the extra capabilities over the D5000 are so worthwhile.  I've still some way to go but I have never expected to hit excellent shots from day one but overall am very happy the D7000.  I've not fine tuned any of my lenses as bad shots I've had I can generally establish through looking at settings and focus points on View NX or Capture NX2.