Singleton Test with JUnit

I have a singleton class like below :

public enum SingletonClassA {
    INSTANCE;

    private final Map<Character, Character> characters;

    // private constructor
    SingletonClassA() {
        Map<Character, Character> aCharMap = new HashMap();
        aCharMap.put('a', 'e');
        aCharMap.put('o', 'u');
        // in order to keep short I erased other puts.
        characters = aCharMap;
    }

    public boolean containsKey(char letter) {
        return characters.containsKey(letter);
    }
 }

And in order to test that I've create only one single object, even though I call more than once, I created a test case with JUnit :

public class SingletonTest {
    @Test
    public void TestSingletonObject(){

        SingletonClassA instance1 = SingletonClassA.INSTANCE;
        SingletonClassA instance2 = SingletonClassA.INSTANCE;
        //Passes
        Assert.assertSame("2 objects are same", instance1, instance2);
    }

    @Test
    public void TestgetInstance(){

        SingletonClassA instance1 = SingletonClassA.INSTANCE;
        SingletonClassA instance2 = SingletonClassA.INSTANCE;
        // Does not pass
        Assert.assertSame(instance1.getInstance('o'), instance2.getInstance('o'));
    }
}

The test passes from TestSingletonObject() which says that those 2 objects are exactly the same. But from the second one, TestgetInstance(), it does not pass.

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My question is: why? Why it does not pass from the second test. I am thinking that even I call the instance methods it should return true because they belong to the exact same object. Am I missing a point?