Regular folks like you and I can get 70 grades too. Please not that I now ONLY send gold coins into NGC. Silver, it seems to me, grades lower.
My experience is dealers and collectors do equally well with NGC, what is more important is quality of coins, conservation and how you submit your coins (in brand new flips out of OMP). I submitted to NGC as a collector club member from 2007-2011, then in the spring of 2011 converted to a dealer member, during both periods I submitted roughly the same number of coins and my 70 yield is very similar.
What often causes confusion among newer submitters is the lack of recognition that "size matters", larger coins 1/2oz+ tend to be much harder to get a 70 grade, say 2-3% probability, while smaller sizes 1/20-1/4oz tend to be easier 6-8%. Also silver tends to be harder than gold for a given size, so for 1oz silver may only yield 1-2% 70s. Also silver coins are often purchased by dealers in bulk (50-300 at a time), as a result the best ones are submitted by the dealers for grading and ones that are left "rejects" often show up on ebay and at shows in OMP. These comments are based on coins dated 1982-2006, I don't deal with newer issues so can't really comment there.
Reason Mirk has done well is he focuses on 1/20 and 1/10 for which 10% yields are possible and he most likely pre-screens coins before submitting, meaning he only sends the best coins in for grading and anything that looks 68ish is sold as is, rather than wasting money on grading.
Crossovers are entirely different matter, prior to 2013 NGC would crossover close to 90% of PCGS coins in 69 grade that didn't show signs of slab toning, hazing, greening and other issues that PCGS slab coins may develop. During the same time PCGS would cross over 60-80% of NGC coins into their slabs. Then last year PCGS grade distributions started to rise (more 69 and 70s awarded) and NGC crossover of PCGS coins began to decrease from close to 90% down to 40%-50%, while PCGS crossover of NGC coins began to increase close to 80-90%.
The new introduction of 70s crossover is too new to understand probabilities yet, but I am not surprised that NGC didn't crossover some of PCGS coins, especially considering NGC coins are trading at 10%+ premiums to PCGS in similar grades.
