The answer is simple: These two never seem to agree about the euro area monetary policy.
Please note that the emphasis in the above sentence is on the word "simple." That assertion of simplicity has to be tested, because in feuding Europe, nothing is simple. But let's start with the simple before largely unsuspected political designs show that the French poet and philosopher Paul Valéry was on to something when he said "What is simple is always false."
So, here is the question: How is it possible to have disagreements about the ECB's monetary policy when the mandate, written in a treaty authored by Germany, says that the bank is solely responsible for price stability within the European monetary union?
Indeed, how is it possible to have constant and publicly aired bitter disputes between German members of the ECB's Council (the bank's main decision-making body) and the rest, when the average inflation rate within the euro area, since its inception, has been 1.8 percent? That should make Germans happy, because the ECB beats the average German inflation of 2.23 percent during the twenty years preceding the introduction of euro coins and notes on January 1, 2002.