Why does the value of the variable 'errno' change from 2 to 22 in the following piece of code?

Question

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>

int main() {
FILE *fptr;

errno = 0;
fptr = fopen("c:\\nonexistantfile.txt", "r");
if (fptr == NULL) {
perror("Error");
fprintf(stderr, "%s\n", strerror(errno));
exit(EXIT_FAILURE);
}

fclose(fptr);
return 0;
}