ConcurrentHashMap 构造器为啥要这么处理？

    public ConcurrentHashMap(int initialCapacity) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException();
        int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?
                   MAXIMUM_CAPACITY :
                   tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
        this.sizeCtl = cap;
    }

tableSizeFor 的道理我都懂，这和 hashmap 一样，但这里为啥不是直接传递 initialCapacity 给 tableSizeFor 呢？

我把两种情况对比了一下，发现好像也没啥规律啊？

1 ==> 1 	 1 ==> 2 ==> 2
2 ==> 2 	 2 ==> 4 ==> 4
3 ==> 4 	 3 ==> 5 ==> 8
4 ==> 4 	 4 ==> 7 ==> 8
5 ==> 8 	 5 ==> 8 ==> 8
6 ==> 8 	 6 ==> 10 ==> 16
7 ==> 8 	 7 ==> 11 ==> 16
8 ==> 8 	 8 ==> 13 ==> 16
9 ==> 16 	 9 ==> 14 ==> 16
10 ==> 16 	 10 ==> 16 ==> 16
11 ==> 16 	 11 ==> 17 ==> 32
12 ==> 16 	 12 ==> 19 ==> 32
13 ==> 16 	 13 ==> 20 ==> 32
14 ==> 16 	 14 ==> 22 ==> 32
15 ==> 16 	 15 ==> 23 ==> 32
16 ==> 16 	 16 ==> 25 ==> 32
17 ==> 32 	 17 ==> 26 ==> 32
18 ==> 32 	 18 ==> 28 ==> 32
19 ==> 32 	 19 ==> 29 ==> 32
20 ==> 32 	 20 ==> 31 ==> 32
21 ==> 32 	 21 ==> 32 ==> 32
22 ==> 32 	 22 ==> 34 ==> 64
23 ==> 32 	 23 ==> 35 ==> 64
24 ==> 32 	 24 ==> 37 ==> 64
25 ==> 32 	 25 ==> 38 ==> 64
26 ==> 32 	 26 ==> 40 ==> 64
27 ==> 32 	 27 ==> 41 ==> 64
28 ==> 32 	 28 ==> 43 ==> 64
29 ==> 32 	 29 ==> 44 ==> 64
30 ==> 32 	 30 ==> 46 ==> 64
31 ==> 32 	 31 ==> 47 ==> 64
32 ==> 32 	 32 ==> 49 ==> 64
33 ==> 64 	 33 ==> 50 ==> 64
34 ==> 64 	 34 ==> 52 ==> 64
35 ==> 64 	 35 ==> 53 ==> 64
36 ==> 64 	 36 ==> 55 ==> 64