Enthalpy, heat of solution test: Calcium chloride vs magnesium chloride in water
It may be caused by the real content of hydrate water in the used calcium and magnesium chlorides. As there is possible disagreement between declared and true water content, due long term reaching equilibrium with air water vapour. I have once seen a glass bottle with reportedly solid $\ce{LiCl}$, sitting on an shelf for years. The content was a clear liquid of LiCl solution.(*)
Both chlorides form multiple hydrates, stable at different humidity or temperature conditions. Generally, with increasing of crystal/hydrate water content, the dissolution enthalpy raises for both salts from negative to positive values. E.g. anhydrous $\ce{CaCl2}$ warms up water when dissolved, $\ce{CaCl2 . 6 H2O}$ cools water down significantly. Note that both anhydrous chlorides are highly hygroscopic.
I suggest to make repetition of the experiments with salts of different origin and short history. Or, heating them well in an oven to get anhydrous stuff.

(*) This hygroscopicity of $\ce{LiCl(s)}$ is used in some humidity meters, measuring equilibrium electric resistence of a soaked fabric.