Copy array's data with memmove function - c

Question

I wrote my implementation of vector ( dynamicly growing array) for integer type. And I was faced with a problem when using memmove and memcpy functions for data reallocation goals. Memmmove and memcpy functions work as not expected. So i replaced it with for and now it works properly. But what is the why memove works this way ?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct vector {
int v_size;
int v_length;
int* v_data;
} vector;
typedef vector *vect;
vect
new_vector(int size) {
vect tmp = (vector*)malloc(sizeof(vector));
tmp->v_size = size;
tmp->v_length = 0;
tmp->v_data = (int*)malloc(sizeof(int) * size);
return tmp;
}
void
add_velem(vect tmp, int elem) {
if(tmp->v_length != tmp->v_size) {
tmp->v_data[tmp->v_length] = elem;
tmp->v_length += 1;
} else {
tmp->v_size = tmp->v_size * 2;
tmp->v_length += 1;
int *new_vector = (int*)malloc(sizeof(int) * tmp->v_size);
memmove(new_vector, tmp->v_data, tmp->v_length); // GOT INPUT LIKE THIS:
// 500, 1, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29,
// ALOT OF ZERO
/*
for(int i = 0; i < tmp->v_length; i++) {
new_vector[i] = tmp->v_data[i];
}*/ // BUT GOT RIGHT INPUT WHEN USING FOR. WHAT IS THE REASON?
free(tmp->v_data);
tmp->v_data = new_vector;
tmp->v_data[tmp->v_length - 1] = elem;
}
return;
}
int
get_vlength(vect tmp) {
return tmp->v_length;
}
int
get_vsize(vect tmp) {
return tmp->v_size;
}
int
get_velem(vect tmp, int elem) {
if(tmp->v_data == NULL) {
fprintf(stderr, "Index out of range!\n");
}else if(elem >= tmp->v_length) {
fprintf(stderr, "Index is out of range!\n");
return -1;
}
return tmp->v_data[elem];
}
void
delete_vector(vect tmp) {
free(tmp->v_data);
tmp->v_data = NULL;
free(tmp);
tmp = NULL;
}
int
main(void) {
vect example = new_vector(10);
printf("lenth of vector is %d\n", get_vlength(example));
add_velem(example, 500);
printf("element %d is pushed into vector\n", 500);
printf("size of vector is %d and\nlength of vector is %d\n", get_vsize(example), get_vlength(example));
int velem = get_velem(example, 0);
printf("elem 0 of vector is %d\n", velem);
for(int i = 1; i < 30; i++) {
add_velem(example, i);
}
printf("length of vector is %d now\n", get_vlength(example));
for(int i = 0; i < get_vlength(example); i++) {
printf("%d, ", get_velem(example, i));
}
delete_vector(example);
return 0;
}