Routerboard throughput performance

Hi all,

got two network arrangements with vastly different performance and want to know why.

First arrangement involves two MT machines; one is an RB532 (ap bridge) and the other is a PC-Router (client). Behind the RB is another PC.
There is a wireless link between PC-Router and RB at 2.4GHz using two CM9 mini PCIs.
The distance between wireless nodes is some 20m and during BTests from the PC-Router via the RB and to the PC I get some 50Mbps. Using IPerf I get some 40Mbps

The second arrangement is indoors between two RB532. There is a wireless link between the RB at 5GHz using two CM9. Two PCs are attached at both RBs. Using BTest/IPerf I can not get more then 27Mbps throughput.
What is wrong here?

In one case the RB is capable of routing through 50Mbps and at there it is not?

thanks in advance for your comment