I'm not exactly sure, because most sources use generic terms like "better system", but it wasn't just the numbers and the infinite supply of ammo, they also had better recon and fire direction.
>>>A component by component examination of American and German artillery shows that almost from the beginning of America’s participation in the conflict the U.S. Army had the superior system. American artillerymen did not try to combat the enemy’s artillery by building bigger guns. The approach from the beginning was to build a better system and it worked. That was clear to thoughtful observers at the time. Viewing the Italian campaign, Field Marshal Erwin Rommel commented, “The enemy’s tremendous superiority in artillery, and even more in the air, has broken the front open.” During the Normandy campaign, Rommel added, “Also in evidence is their great superiority in artillery and outstandingly large supply of ammunition.” By any reasonable standard, especially during the latter part of World War II, the American artillery arm was very clearly superior to that of the Germans.<<<
>>>The effectiveness of American artillery, even at this early stage of American involvement, impressed Rommel. In an 18 February 1943 letter to his wife, he described the fighting in and around what American historians have called the Battle of Kasserine Pass. In part he commented “an observation plane directed the fire of numerous batteries on all worthwhile targets throughout the zone.”<<<
armyhistory.org
And that's not even considering the proximity fuzed shells introduced in the Battle of the Bulge.
The "outstandingly large supply of ammunition" is why I think SF should give a +10% supply usage, which you have to compensate using Logistics company.
The soviets had the opposite situation. Although they had tons of artillery guns, they often had little ammo.