Deliberate mistake: John yells, "I've got it! I've got it!" jumps onto his bed and we see the length of John's one blanket, mattress and bed frame near the other furniture. In the next two shots as John runs across his bed, the blanket at the foot of the bed is not only pulled all the way out, using what hung over the foot of the mattress in the previous shot, but another blanket is added for extra length. Most blatantly apparent is the span of this mattress and bed frame, to increase the running distance, in relation to the furniture around it.
Picture added by Super Grover