Why does IdentityHashMap use 2/3 as the load factor instead of 0.75?

public class IdentityHashMap<K,V>
    extends AbstractMap<K,V>
    implements Map<K,V>, java.io.Serializable, Cloneable
{
    /**
     * The initial capacity used by the no-args constructor.
     * MUST be a power of two.  The value 32 corresponds to the
     * (specified) expected maximum size of 21, given a load factor
     * of 2/3.
     */
    private static final int DEFAULT_CAPACITY = 32;
    
    // omitted
}

From jdk/IdentityHashMap.java at jdk8-b120 · openjdk/jdk · GitHub

Update

I also found that ThreadLocal.ThreadLocalMap also uses 2/3 as the load factor, It also uses linear probing to resolve conflicts.

/**
 * Set the resize threshold to maintain at worst a 2/3 load factor.
 */
private void setThreshold(int len) {
    threshold = len * 2 / 3;
}

From jdk/ThreadLocal.java at jdk8-b120 · openjdk/jdk · GitHub