omnet++ M/M/1 queue length

[Fabio Musso]

I try to simulate MM1 queue classic with infinite queue, exponential inter-arrival and service rate.

 My problem is when a calculate through statistics the average length of queue. 

Theoretically, using arrival rate lambda = 1/20 and service rate mu = 1/5, i should experiment a mean queue value of 1/6 given that rho = lambda/mu = 1/4 and E[number of users in queue] = rho^2/(1-rho) = 1/6. 

My simulation of 5000s of sim-time-limit give to me a value of 1/2. I don't understand why, because all other statistics (response time,waiting time,ecc.) are right.

Thanks in advance.