Ezreal true shot barrage on longer cooldown than on tooltip

I played an Ezreal game and I used my ultimate on a minion wave to clear it and my ult when on a 2-2:30 minute cooldown instead of a 66 second cooldown as indicated on the tooltip. i hadnt payed to much attention earlier in the game but i believe it only happened the one time( it was also rank 3-lvl 16 ultimate).