mass balance

Hello

I am a user of Comsol 4.3b. I have a very simple geometry of some channels. I just need to see the flow
distribution between different channels. The flow is laminar and the boundary conditions are inlet mass flow
and outlet pressure.
I used laminar flow phisics and I tried to calculate the mass flow by intergration of U.rho through different
surfaces.

my problem is that although the soloution is converged but I see there is no mass ballance between the inlet
and the outlets.

inlet mass flow: 56 g/min
outlet mass flow: 56.80 g/min

I also refined the meshes but it did not help. I appreciate if any body can tell me what is wrong with my
model. The file is attached here.