CollectionViewIndexPathForPoint在单元格中断时返回nil

Question

下面的lldb打印输出显示indexPathForItemAtPoint:在细胞分裂上返回nil。这是为什么？

(lldb) po [self.collectionView indexPathForItemAtPoint:CGPointMake(0, 83)]
<NSIndexPath: 0x17824540> {length = 2, path = 0 - 0}

(lldb) po [self.collectionView indexPathForItemAtPoint:CGPointMake(0, 84)]
 nil
(lldb) po [self.collectionView indexPathForItemAtPoint:CGPointMake(0, 85)]
<NSIndexPath: 0x17824500> {length = 2, path = 0 - 1}

(lldb) po [self.collectionView indexPathForItemAtPoint:CGPointMake(0, 169)]
<NSIndexPath: 0x17824500> {length = 2, path = 0 - 1}

(lldb) po [self.collectionView indexPathForItemAtPoint:CGPointMake(0, 170)]
 nil
(lldb) po [self.collectionView indexPathForItemAtPoint:CGPointMake(0, 171)]
<NSIndexPath: 0x17824510> {length = 2, path = 0 - 2}