Actual thickness of permafrost vs theoretical maximum possibly permafrost thickness I came across a question in my notes about why the actual thickness of permafrost is much less than the theoretical maximum possible permafrost thickness value.

The actual permafrost measure on sites were much less thick than the maximum possible thickness.

So the actual permafrost thickness had values ranging from 0-15m. But the maximum possible permafrost thickness had values froom 75, to 375m. We determined the maximum possible thickness by using the formula $z=(T_s)(k/q)$. With $T_s$ which was -3 and we took the absolute value of this so 3. Then K was the thermal conductivity $W/m$ and Q was the heat flux which was $.04w/m^2$. So why was the actual thickness much less than the theoretical max possible? Clearly the max possibly max is going to be higher than the actual thickness because this gives a end range for the thickness of the permafrost but besides that I am not quite sure  

Edit: as an example, the following values are for Newfoundland and Labrador.  The maximum possible thickness was calculated by that equation I gave up above.

Material, Max Possible Thickness, Actual Permafrost Thickness:
granite, 75m, 0m
sandstone, 150m, 1m,
glacial till, 113m,1m
esker gravel, 188m,0m
marine silt, 375m,15m
river sand, 225m, 0m
fen peat, 300m, 10m
I could not recover my account so here is some additional information for you. This area is Newfoundland and Labrador.  The maximum possible thickness was calculated by that equation I gave up above

Material, Max Possible Thickness, Actual Permafrost Thickness:
granite, 75m, 0m
sandstone, 150m, 1m,
glacial till, 113m,1m
esker gravel, 188m,0m
marine silt, 375m,15m
river sand, 225m, 0m
fen pea,t 300m, 10m