Input and output flow mismatch

Hello, how are you doing?
I´m simulating a simple flow system channel with an input and three outputs. When i compute the input flow trough a boundary integration at each one of the input and outputs i realize that the total input flow does not match the total output flow. In my case, the total output flow is higher than the input flow.
Can someone tell me why this should be happening? How can i correct this problem?

Thank you very much