The problem is that the graph you mentioned is gain vs length of the amplifier. The 1480 nm pump region is more POWER EFFICIENT not necessarily higher gain.
Look at the graph here that compares the pump power to the experienced gain. The 1480 nm pump outperforms the 980 nm pump. Good question though I was confused too! The key here is that the 1480 nm is more efficient, but NOT higher total gain.
Regards