Ok, I understand. So, you were transfering data from one laptop to another. One laptop was associated to the 11a side of an AP and the other laptop was assoicated to the 11g side of the AP? So, why the slow throughput? Well, you are doing a wireless to wireless transfer. One wireless link can only give you 20Mbps, so when you are transferring from one to another, you should never see much more than that. To test what want (I think) you need three machines. Wire one of them to the AP. Make this the FTP server. Then, use the other two machines to pull the FTP, one on 11a and the other on 11g.
Does this help?