multicomponent gas calculation imbalance？
 

I use multicomponent model to calculate the mixing of three gases. Transport Equation is selected. But after two days' calculation, the result of the Mass Function imbalance? Why please?

the result:

+------------------------+ C2H6-Mass Fraction | +------------------------+ Boundary: fint 6.4003E-05 Boundary: out -3.0047E-05 ----------- Domain Imbalance : 3.3956E-05 Domain Imbalance, in 53.0538 %

+------------------------+ CH4-Mass Fraction | +------------------------+ Boundary: fint 3.5986E-04 Boundary: out -1.6894E-04 ----------- Domain Imbalance : 1.9092E-04 Domain Imbalance, in 53.0538 %