Why is the tension developed in muscle due to tetanic contraction more than the tension developed by contraction due to single twitch?

Usually, the maximum tetanus tension is from 1.2 to 1.8 times greater than the maximum tension during a twitch. Within the muscle, many  elastic structures, connected in series with the contractile elements, are stretched during contraction. The attachment of the muscle fibres to the tendons at the end of the muscle and the attachment of the thin filaments to the Z line contribute to this elastic component. In single fibres, however, most of the  elasticity of the series of elastic elements is contributed by the actin-myosin cross bridges themselves. Full maximum tension is not apparent at the end of the muscle until the contractile elements have shortened enough to stretch the elastic elements—somewhat like taking up the slack in a rope before a pull on one end can be felt at the other end. In a twitch, the activity of the muscle is so brief that the contractile elements cannot extend the elastic elements completely before relaxation begins; as a result, the tension at the ends of the muscle does not reach the maximum possible level. During a tetanus, on the other hand, the activity of the contractile elements is maintained, and they can eventually shorten enough to extend fully the series of elastic elements. When this has been accomplished, the maximum tension is apparent at the ends of the muscle.