Because theres only 1 size of Silent Coat mat (375x265). And you've used what looks to be about 10 sheets when theres 40 in a 'box'.
Is it possible that you bougtht one of the door liner packs with only 8 sheets in?
Silent Coat 2mm Mat Door Pack
I think they are designed to do maybe a pair of car doors.
When you said you had used 2 full boxes I imagined you had completely filled the void with silentcoat and added about 20Kg to the rear panel!