Why does calorimetry experiments on combustion usually give you lower values than expected values

Question

Why does calorimetry experiments on combustion usually give you lower values than expected values

Answer 1

There can be various reasons, but the most common is the calorimeter itself.  Generally, unless it is an industrial type, the calorimeter is not totally insulated, and thus allows some heat to escape. This heat is then not recorded as in increase in temperature, and the heat of combustion is consequently underestimated.