For those of us that have the D600 or D7100, the number of cross sensors and line sensors at higher apertures drops a lot faster than the top of the line D800 and D4. So while the D7100 has the Multi-Cam 3500 focus system, it is the DX version. On the D7100 that means above f/5.6 you only have a single sensor that operates fully - and it operates as a cross sensor as well. Yes it's true that the D7100 has 51 point AF and AF to f/8, but it's just one sensor and not the same as the D800 or D4 which have a lot more sensors at f/8.
The good news is we are seeing this openly communicated. Earlier cameras had a similar design, but did not communicate as clearly the difference across the sensors at higher apertures.