What an intriguing question! I think that the answer lies in the intent of the person. Sometimes a person was so overcome with a desire to give something directly to God, that God gave us the opportunity to bring a completely voluntary Shelamim sacrifice. However, to bring a Shelamim correctly means not to bring it at the expense of giving extra to Tzedaka (charity), but rather in addition to giving extra charity. May we merit to see the building of the third Beit Hamikdash (Temple) and the ability to bring sacrifices again speedily.