Investigation into the Enthalpy Change of Combustion of Different Alcohols - query

[mamakosj]

When conducting an experiment to test the relationship between the enthalpy changes of different alchols, I came across something which i couldn't explain. Taking propan-1-ol and propan-2-ol as an example, I found that propan-1-ol had a higher enthalpy change. Could someone explain to me why this is so. (the same thing happened for butan-1-ol and butan-2-ol)

 

Although I have no precise knowledge to base my hypothesis on, I would have expected this relationship to be the other way round. Please help.