I feel it comes down to trust.
I'm not familiar with Mike, but Dean has been around a long time. Even written books about cards.
When you are buying on EBay, you don't really know the person selling or their product. But, if you have purchased from Dean, and I assume Mike, you have a better idea of what to expect of your purchases.
You can even go to Dean's website and watch the training video for his employees on grading.
Like I said, I think it comes down to trust. Some people are willing to pay a bit more for the piece of mind.