Difference in iterating over value type for std::map vs std::unordered_map

The following program iterates over an unordered_map trying to find the best element but doesn't quite return the expected result:

#include <iostream>
#include <unordered_map>
using namespace std;

struct Item
{
    int val;
};

int main() {
    unordered_map<int, Item> itemMap;
    itemMap[0] = {0};
    itemMap[1] = {1};
    itemMap[2] = {2};
    itemMap[3] = {3};
    const Item* bestItem = nullptr;
    int bestVal = -1;
    for (const pair<int, Item>& item : itemMap)
    {
        if (item.second.val > bestVal)
        {
            bestVal = item.second.val;
            bestItem = &item.second;
        }
    }
    cout << "best val: " << bestVal << " best item: " << bestItem->val;
    return 0;
}

ideone

Running this program prints out:

best val: 3 best item: 0

This seems to occur because the value_type of a unordered_map is std::pair<const Key, T> but we are iterating over const pair<int, Item>. Sure enough changing this to const pair<const int, Item> results in:

best val: 3 best item: 3

However if we change the type of itemMap to std::map:

map<int, Item> itemMap

Then it doesn't matter if we iterate over const pair<int, Item> or const pair<const int, Item> we get the same result:

best val: 3 best item: 3

Even though the value_type of std::map is still std::pair<const Key, T> . But why?