Why the process return value is different from what the code returns?

Question

Consider this code:

int main() { return 10'000; }

I compile it and run it:

$ g++ main.cpp -o p
$ ./p
$ echo $?
16

Why the output of $? is 16? I also tried with Online Compiler too:

• https://wandbox.org/permlink/ovI5sb9xlh7VpYne

And the process return value is 16. What I'm missing?

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My environment:

• OS: Arch Linux x86_64 Linux 5.13.10
• Compiler: GCC 11.1.0
• Compiler Additional Flags: -Wall -Wextra -Wpedantic