Dependencies between throughput and TCP window

Hi, I have observed an interesting throughput pattern when testing two different WiFi access points & Windows7. In both cases the radio link is perfect. FTP download throughput is always around 50% lower on one of the APs (40 vs 20Mbps).

Both APs show no dup-acks or retransmissions. Link is just perfect. The difference is that when the faster AP is used TCP window keeps increasing all the the to 1.5Mbytes, while on the slower AP the TCP window increases to 200kbytes and stays at this level.

Other differences are: Fast AP - TCP SYNC time around 40ms - Maximum segment size - 1442

Slow AP - TCP SYNC time around 30ms - Maximum segment size - 1414

Any suggestions?

Cheers, Amebcia