Under the assumption that you are running the users with the same settings (CQI, number of antennas, etc), then the gain you see here is probably due to the time diversity harvested by channel coding. Since the codeword occupies the entire subframe, then the highly time-selective channel will affect the different parts of the codeword with different fading amplitudes. In terms of channel decoding, it means that the increased time-selectivity (due to the higher velocity) will decrease the chances that the entire codeword is in deep-fade, and therefore the surviving parts of the codeword can be used to correct the errors in the other deep-fade parts, loosely speaking.
So apparently here, the intercarrier interference due to the Doppler spread from the high velocity is not so serve here. Also, if you enable realistic channel estimation, i.e., “PilotAided”, then probably the gain will decrease.