Program A, with global variables x and y:

#include <cstdio>
#include <vector>
using namespace std;

const int maxn = 3000000 + 10;
int dp[maxn], n, maxy;
int x, y;        // <-- Global variables `x` and `y`
vector<int> a[maxn];

int main() {
    freopen("demo.txt", "r", stdin);
    freopen("demo.out", "w", stdout);

    scanf("%d", &n);
    for (int i = 1; i <= n; i++) {
        //int x, y;
        scanf("%d %d", &x, &y);
        a[y].push_back(x-1);
        maxy = max(maxy, y);
    }
    for (int i = 1; i <= maxy; i++) {
        dp[i] = dp[i - 1];
        if (a[i].size() == 0) continue;
        for (int j = 0; j < a[i].size(); j++) {
            int u = a[i][j];
            dp[i] = max(dp[i], dp[u] + i - u);
        }
    }
    printf("%dn", dp[maxy]);
    
    return 0;
}

Program B, almost the same with program A, but with local variables x and y (x and y can be considered as the two endpoints of an interval that $x < y$).

#include <cstdio>
#include <vector>
using namespace std;

const int maxn = 3000000 + 10;
int dp[maxn], n, maxy;
//int x, y;
vector<int> a[maxn];

int main() {
    freopen("demo.txt", "r", stdin);
    freopen("demo2.out", "w", stdout);

    scanf("%d", &n);
    for (int i = 1; i <= n; i++) {
        int x, y;         // <-- Local variables `x` and `y`
        scanf("%d %d", &x, &y);
        a[y].push_back(x-1);
        maxy = max(maxy, y);
    }
    for (int i = 1; i <= maxy; i++) {
        dp[i] = dp[i - 1];
        if (a[i].size() == 0) continue;
        for (int j = 0; j < a[i].size(); j++) {
            int u = a[i][j];
            dp[i] = max(dp[i], dp[u] + i - u);
        }
    }
    printf("%dn", dp[maxy]);
    
    return 0;
}

Compile instruction (with -O2 optimized):

g++ demo.cpp -o demo -O2 -std=c++14

Compile information:

Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=C:/mingw64/bin/../libexec/gcc/x86_64-w64-mingw32/8.1.0/lto-wrapper.exe
Target: x86_64-w64-mingw32
Thread model: posix
gcc version 8.1.0 (x86_64-posix-seh-rev0, Built by MinGW-W64 project)

Those two programs get the different results with the same input!

With the same input data, I’ve watched the variables of x, y, maxy, a[i], and all of them got the same data. And the array element dp[i] got the different data. I don’t know why, the variable dp[i] and x, y are unrelated!

The big input data comes here: https://ww0.lanzouq.com/iesib26alt6f

It’s a txt file, with about 150001 lines.

BTW:
This program is a solution to this question, However, you can ignore this information. The issue I’ve gotten seems to be no relation with that question.