[ns] Throughput value difference in 802.11

[Иван]

Hi!
I am using 802.11 Ext model and when I plot the througput in Xgraph it turns 
out to be different depending on how many traces (e.g. MAC trace, Agent trace, 
PHY trace) are turned ON. But the thoughput can't change. 
So the question is how can I know where there is   a REAL throughput value?
Thanks in advance!
Ivan.