ISO/DIS 2240 slides speed evaluation - confusion

Hello,

I've beeen searching quite a long time for the answer. In ISO/DIS 2240
there's is a color reversal speed defined as square root between
Dfog+0,2 and tangent to shoulder or 2,0D point. Only the 2,0D point
gives right speed for low and high sensitivity slides. Everything is
good with this bonduary condition and with film speed equation:

ISO speed = 10/10^Hm where Hm=0.5*(logHs+LogHt)

Hm - sensitivity point;
Hs - shoulder point (two possibilities, see above);
Ht - toe point (0.2+Dfog);

But this is only a synthetic speed based on HD curve. If you now take
a photograph of a gray card then its density on film is somehow
1.25-1.3D (0.75*gamma 1). From speed equation density from a given
exposure is lower - below 1,0D!

So can somone clairify this issue - the difference between luxmeter
exposure on gray card and synthetical Hm point for speed calculation?
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