DWA-160的iperf測試結果疑問

各位好：

最近入手dwa-160，其連線速度標榜300M，怎麼用iperf卻測出下面這種數據？

[ 3] local 192.168.1.3 port 63049 connected with 192.168.1.6 port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-10.0 sec 47.5 MBytes 39.8 Mbits/sec
[ 3] 10.0-20.0 sec 48.1 MBytes 40.4 Mbits/sec
[ 3] 20.0-30.0 sec 50.4 MBytes 42.3 Mbits/sec
[ 3] 30.0-40.0 sec 47.1 MBytes 39.5 Mbits/sec
[ 3] 40.0-50.0 sec 49.5 MBytes 41.5 Mbits/sec
[ 3] 0.0-58.9 sec 286 MBytes 40.8 Mbits/sec

請問這樣的速度正常嗎？
測出的Bandwidth是不是應該接近300Mbits呢？
測試環境：win7(iperf client) <-> MacBook Air(iperf server)
Win7顯示無線連線速度為300Mbits。
先感謝指導！