mass flowrate is non conserved

Dear all,

I was trying to simulate the water flow into a system with two inlets and three outlets and I used the ''surface integration'' io order to compute the total mass flow rate and I found that the total outgoing mass flow rate is higher (around 5%) than the incoming mass flow rate.
Thus, I decided to do a simpler case, a cylinder representing an injector, and comparing the numerical results in terms of outlet velocity with the experimental ones. Numerical results agree with the experimental but I still found that the mass flowrate through the exit surface is higher (around 1.5%) than the inlet mass flow rate.

Could it be due to the mesh resolution or not?

Thank you in advance for your help.

Antonio