How to calculate the normal heat flux on a boundary

Hi everyone. I´m a beginner with COMSOL and I don’t understand how it calculates the normal heat flux on a boundary.

In order to evaluate how to derive values in the Results section, I have followed the Heat sink tutorial included in COMSOL 4.2a and once I get the results of the simulation, I calculate the Surface Integration of the Total normal heat flux (nitf.ntflux) on surfaces 2, 3, 4 and 5. These surfaces have Thermal Insulation boundary conditions by default; hence, I expected to obtain 0 W on each surface. However, the values calculated by COMSOL are nitf.ntflux = 0.01353 W (surface 2), nitf.ntflux = -0.03251 W (surface 3), nitf.ntflux = 0.03545 W (surface 4), nitf.ntflux = 0.01363 W (surface 5) and nitf.ntflux = 0.0301 W (surfaces 2, 3, 4, and 5). In this model, there is only a heat source of 1 W on surface 8; therefore, the calculated Total normal heat flux on the isolated surfaces is higher than 3% of the supplied power.

Are these results misunderstood? Or there is something wrong with the model? Or…

Could anyone help me to understand these results?

Thank you and regards.