By the time the child has been led into the bedroom, he has forgotten the transgression. He associates the spanking with the bedroom, not the deed done in the living room. The same loss of association occurs when you delay spanking to grill the child with long lectures. He forgets the transgression and thinks he is getting a spanking for being bored with the speech. You don't need to make a lengthy ritual out of it.
Some parents try to turn discipline into a revival meeting, complete with altar calls.
Likewise if you are not consistent, and you allow the child to occasionally get away with his lack of self-control, you are allowing a seed of dark hope to grow in his imagination. You need to compare the child's desire to indulge to the desire of an alcoholic. It is not rational. It is idiotic. Just the smell of indulgence will drive a child to absurd lengths.