I am more than just an educated consumer. I have gone through GIA classes, and while I might not be a vendor or work in the jewelry field, I actually AM educated. I have seen many stones accurately graded, or that could be subjectively accurate, anyway. I have seen stones that I think were softly graded by them. I have seen some that have been harshly graded by them.
It is far from libelous to state the facts. They operate their own grading facility for the stones they sell. That does NOT make them reputable, that actually makes them quite biased in their own favor. So yes, that can very well be dishonest and misleading. They really can call it whatever they and their in house "experts" who very well may be trained by someone like GIA, but they could also just be well trained salespeople who over time learned how to identify. I have heard but no one can seem to prove it, that they do hire GIA trained gemologists to grade. They just expect you to just accept their grading. I would prefer that my stone came with a reputable report, from the likes of GIA or AGS, even EGLUSA, even if it came from Tiffany.
They don't base their reputation on their honesty. And just because they're extremely successful, doesn't always make someone reputable. Those auto warranty companies were also wildly successful, and now most are gone because they've been caught as been con men. Tiffany bases it on their opinion of their brand and how they can persuade the public to consider them reputable. They might feel it's honest, some people may disagree. They are in the business of selling a "lifestyle". And many people want that lifestyle. A lot of that, much like every other brand, is marketing and public opinion. Because you are a happy Tiffany's customer, you believed their marketing and you live your life along their lifestyle.
It's not hard to double your price though that's for sure!