Interesting... so I keep getting deeper into this research as I wait for my rings to come in
and running all four diamonds through this calculator
http://www.jamesallen.com/diamond-education/diamond-cut/
and the GIA Diamond Cut Grading System chart, they all DO in fact fall within Ideal or Excellent cut grades. Tiffany's (it seems!) is telling 100% truth about "all of their diamonds being excellent cuts"! At least in my experience!
This includes my diamond which only scored 3.9 on the HCA... so what does that mean? My diamond fell in the 'ideal' grade. So why isn't it TIL range on the HCA???
Well, GIA didn't start "grading" cut til recently, late 2000's I believe. And even then, they don't really grade cut. They really only started doing it on their reports because they had to to stay alive. The marketing of ideal cuts and H&A has kind of forced them into it and they now have to compete against a lab like AGS who specializes in cut grading.
GIA might be very reputable in the other characteristics but as far as cut, they kind of have their own system and one that is very fluid right now. I thought it was based on Tolkowsky standards but now I think it's kind of a mix and match of a few things and it's quite broad and doesn't seem to have uniformity. So far it seems like a lot of estimating to me. I know it's based on several characteristics that the HCA does not really factor in, star facets, painting, etc. They've been "researching" it for like 20 or 30 years now though. I will be very curious where it ends up. AGS has a pretty solid set of angles and characteristics of what they deem ideal, based on angles and percentages and symmetry.
Also, GIA doesn't assign IDEAL as a cut quality grade, their highest is Excellent. To me, they assign an awful lot of stones to Excellent that to AGS or even the HCA scale would not really qualify as such. AGS does use IDEAL, though. They pioneered that specific grading report system, really. And they developed their system for grading cut on a SARIN machine and the results it would produce. They also developed the ASET scope which helped identify the facets that weren't at the most ideal angles for the best light return and identify facets and cutting patterns that might allow for more "leakage", areas that the light isn't reflected back, and instead just goes right through the stone. Tiffany grades their own, sometimes based on GIA/AGS certs on stones they purchase. So they might also use IDEAL in house but I have no idea where they pull that from and what criteria they use to assign IDEAL to something.
Garry Holloway developed HCA to help weed out stones that were likely not great performers in terms of angles and numbers. The HCA factors in spread beyond just brilliance, fire and scint, and it's desirability to the buyer. If you can spend slightly less on a smaller stone that is cut phenomenally well and looks as big as one that is less well cut and doesn't look as big as it should, obviously you will want the one that looks amazing and is slightly smaller in number but looks as big as a bigger stone. If it's cut properly it's got edge to edge brightness, meaning it will look spreadier.
So short answer: while your stone might have said Excellent Cut via GIA, the angles that the HCA is developed to accept as ideal and produce the most light return will return a less than favorable score because it falls outside of the HCA cut parameters he has set.
His little system though is not the end all be all at diamond buying. I have seen some AMAZING stones that fell in the 3 and 4 range. I have also seen some DOGS in the 1 range. It isn't the ultimate decider, it just helps you identify which ones to view with your eyes. I have also seen stones in person that are AGS Ideal and pass through all the machines with amazing numbers and results and then next to a GIA Excellent that got less than amazing numbers and results look less than awesome. Just because it's good on paper, doens't mean it will be good to the eyes, and vice versa. And just because some computer tells you it's the best damn thing ever, doesn't mean you're gonna love it when you see it.
Also--AGS has a pretty scientific system for picking stones out as IDEAL, as does the HCA. But grading as a whole, and how GIA would generally do so, beyond facetware, is subjective. Just like taste. Everyone likes different things.
I'm dying to know what the 2.10's score is but I think I'm driving my SA nuts!