WSPR reports vs VOACAP predicted values

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f6irf
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WSPR reports vs VOACAP predicted values

I have posted a few charts on my blog http://f6irf.blogspot.com/ which show quite important differences between the WSPR reported values and the VOACAP predicted values. It is based on temporary data (April 1st to 20 on 30m) on a particular path (F6IRF>W1BW). While the bands openings/closures seem to match quite closely the predicted values, there is an important difference (16~20dB), between the predicted SNR and the reported ones. I have used various "required reliability", methods (20, 22 and 30), absorption model, coefficients and SSN values, but still the difference remains... Of course there is a certain number of uncertain parameters, but I do not believe that my vertical has 16dBi gain !
I will repeat the same with other pathes, with a higher number of spots, but in the meantime I would be interested to know if anybody has already done such comparisons, or has an ideas on what could cause such an offset !
Regards - Patrick