How is it that (temperature) heat makes some seasonings and foods (spicy) hotter, but mellows others?
I expect that you have already considered the most probable answer. For some spices, the aromatic compounds are released by some heat, but are not destroyed. Thus their effect is heightened. For other compounds, even a small amount of heat will destroy them and thus reduce their effect - see Mischa's comment.

I believe that the difference essentially lies in at what temperature a flavoursome compound starts to degrade.