# Heat capacity of mixtures is higher than their pure contituents?

I'm currently studying the effects of how changing mixed-refrigerant compositions effects their heat capacity. Surprisingly, my data suggests that some combinations of mixed-refrigerants have heat capacities larger than their pure constituents. This seems to violate Kopp's Law which states:

The molecular heat capacity of a solid compound is the sum of the atomic heat capacities of the elements composing it; the elements having atomic heat capacities lower than those required by the Dulong–Petit law retain these lower values in their compounds.

The mixed refrigerant compositions include: methane, ethane, propane and nitrogen. The graph below shows average, minumum and maximum heat capacity values over the temperature range from $\pu{130 K}$ to $\pu{280 K}$ and pressure range from $\pu{1 bar}$ to $\pu{100 bar}$.

Is there another possible explanation as for why a combination of the above components would have a higher capacity than their pure constituents? I was thinking along the lines of molecular interactions and deviations from the ideal gas law.

Here is a graph of my findings:

Note: I am using a gas physical property software for these calculations.