我现在正在进行“Rust by Practice”练习。在“Collection Types〉HashMap”一节中有一个例子：

use std::collections::HashMap;
fn main() {
    let mut map: HashMap<i32, i32> = HashMap::with_capacity(100);
    map.insert(1, 2);
    map.insert(3, 4);
    // Indeed ,the capacity of HashMap is not 100, so we can't compare the equality here.
    assert!(map.capacity() >= 100);
    
    println!("Capacity #1: {}", map.capacity());

    // Shrinks the capacity of the map with a lower limit. It will drop
    // down no lower than the supplied limit while maintaining the internal rules
    // and possibly leaving some space in accordance with the resize policy.

    map.shrink_to(50);
    assert!(map.capacity() >= 50);
    
    println!("Capacity #2: {}", map.capacity());

    // Shrinks the capacity of the map as much as possible. It will drop
    // down as much as possible while maintaining the internal rules
    // and possibly leaving some space in accordance with the resize policy.
    map.shrink_to_fit();
    assert!(map.capacity() >= 2);
    
    println!("Capacity #3: {}", map.capacity());
    
    println!("Success!");
}

我运行上面的程序得到以下输出：

Capacity #1: 112
Capacity #2: 56
Capacity #3: 3
Success!

我的质询如下：
我们将一个容量为100（元素）的空HashMap分配给变量map。然后我们将两个元素插入到HashMap中。因此，整个HashMap的长度为2个元素。所以我们有98个地方可以插入更多的元素，对吗？那么为什么Rust编译器在这里将容量扩展到112？
同样，当我们将HashMap的容量缩小到50时，真实的容量似乎是53，尽管我们仍然只有2个元素。
第三个输出也是如此。有人能给我解释一下这是怎么回事吗？