KBS TGI shaft flexes?

[G56788]

I was looking at a driving iron and one of the shaft options for a stiff flex was a KBS TGI 80 gram shaft. According to the chart on the KBS website, I should be hitting a TGI 90-gram shaft based on my 6 iron swing speed. The salesman said I would be “fine” with the 80 gram when I asked about the chart. When I researched it further, the 90 gram is apparently a regular flex shaft? The 100 gram shaft is a stiff flex but the chart does not recommend that for my swing speed.
 

Does anyone know how KBS fits the TGI and why a heavier shaft with a weaker flex is recommended for a higher swing speed?

[Golfweasel]

KBS fits based on ss of your 6i and which iron hosel diameter you have  

if your iron is Parallel tip, shaft weights range from 50g - 80g (10g increments). 80 being most stiff. 
If your iron is Taper tip, shaft weights range from 60g-110g (10g increments). 110 being most stiff. 
check out their website for more info.