Difference in throughput when Station is connected to Laptop

Hi All,

I have setup a link at 500m distance, I am using rb433 router board with compex card.

There are two scenarios:

Scenario 1. when Base is connected with PC and Satellite is connected with Laptop, and if i run a bandwidth test using winbox on base I receive a throughput of 14 Mbps.

Scenario 2: But when Base is connected with PC and Satellite is not connected with Laptop or PC, and then if i run a bandwidth test using winbox on base I receive a throughput of 18 Mbps.

Can anyone please explain the difference.
And how can i achieve the throughput of 18 Mbps.

Setting on Base - AP Bridge (WDS enable)
Station - Station WDS

Regards,
Prakhar