GigaLynx Capacity Tx/Rx Diffs

I notice that an 18G GigaLynx shows double the capacity on the Rx, even though the modulation is lower

Local Link: Locked RSSI: -50.50 MSE: -38.70 Rx: QAM1024
Remote Link: Locked RSSI: -46.90 MSE: -26.90 Rx: QAM64

Tx Capacity(SM/LG)(Mbps):387.00/ 246.00
Rx Capacity(SM/LG)(Mbps):632.00/ 401.00

Can you explain that to me?  Do we need to perfect the aim of one of the polarities?

Thanks,

Ruth