Help with ROT-13 C encryption

Question

Some letters of the encryption are passing from 'z', and going to symbols in the ASCII table, why is this occurring?

#include <stdio.h>  // CRIPTOGRAFIA ROT13 
#include <stdlib.h> // A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z
#include <string.h>

int main()
{
    int i;
    char Palavra[255];

    printf ("Digite a palavra que deseja criptografar: ");
    scanf (" %s", Palavra);

    for (i = 0; i < strlen(Palavra); i++)
    {
        if (Palavra[i] >= 'a' && Palavra[i] <= 'z')
        {
            Palavra[i] = Palavra[i] + 13;
        }
            if (Palavra[i] > 'z')
            {
                Palavra[i] = Palavra[i] - 26;
            }
    }   

    printf ("%s\n", Palavra);

    return 0;
}

Answer 1

Let’s make it simple. What you should do is you should add up to half of the alphabet 13, if you pass that part you should subtract 13. Isn’t it simpler? I took advantage and improved performance by eliminating a loop needlessly.

#include <stdio.h>

int main() {
    char palavra[255];
    printf("Digite a palavra que deseja criptografar: ");
    scanf(" %s", palavra);
    for (int i = 0; palavra[i] != '\0'; i++) {
        if (palavra[i] >= 'a' && palavra[i] < 'n') palavra[i] += 13;
        else if (palavra[i] >= 'n' && palavra[i] <= 'z') palavra[i] -= 13;
    }   
    printf("\n%s\n", palavra);
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.