Just thought i'd mention - multiple barrage shots can be spread out on successive hits. the only stipulation is that the new blast template must be placed in contact or overlapping a previously placed blast template.
So i think that the second large blast could have been placed in contact with the first (covering some models) and the third blast would have been quite near the intended location if placed touching the second one.
It's one of the good points about having multiple quad-mortars so you can creep your hits back to where you wanted them!
edit: Just finished watching - really enjoyed it guys!
Check out the mechanicum before you start buying marines....