fuel cell stack block bug?

I created simple model-fuel cell stack with active resistance which is constant. I modelled with hydrogen 99,91, oxygen 21.* But* connected scope to measurement output and scope showed that "02 utilization,%" is constant and value 60%. I checked also example on help of matlab. There is set oxygen utilization -21% of fuel cell stack at help, but results of o2 utilization on help shows that O2 utilization is ~60%. Is it bug?

I checked signal variation of oxygen of fuel cell stack and set input signal from 1-100%. But "fuel cell stack " measurment signal again shows constant value of oxygen utilization and it is 60%. Is it bug of fuel cell?