In PMP system: how can I improve uplink rates to 4Mbps?

In PMP system, when I test throuputs AP-SM,I found however I adjust downlink percent, for example :10%,20%,or 30%,the uplink rates is also low,only less 2 Mbps.

But in PTP system,when I set a downlink percent 25%, BH-BH test is : downlink rate is 1.8 Mbps,and uplink rate is 5.4Mbps.

Can somebody tell me the reason? Thank u very much.