A supported shot while seated is one of the most stable shots available to a shooter.
Yes it is and that might very well explain the missed first shot. There are a number of factors that indicate that first shot was the most difficult of the three. Given the extreme downward angle of the shot, I doubt he could have maintained a seated position nor would the boxes been much use as a rifle rest. He would have been tracking his target coming out of a sharp 110 degree curve which means the limo would have been moving in an arc until it straightened up. The target would have been moving across his line of fire as opposed to down the line of fire for the subsequent shots.
It makes one wonder why he even took that shot. Perhaps he was a disciple of Wayne Gretzgy who said "You miss 100% of the shots you don't take."
(Yes, I know. This was long before anybody heard of Wayne Gretzky who was less than 3 years old at the time.)