its that chunk of lead hidden in the forestock. Rather have a slight heavier rifle that's balanced then a ligher one that wobbles. Never seen a HW55MM so can't say if it came with the Walther's forestock weight.
As Gaines mentioned, those aiming for supreme accuracy favor more weight then less. Check out the extra lead in the butt section of the LGV-T used in several world champioinship level events. It it just under the 11 lbs. maximum UIT (now ISSF) total weight limit.