Bash: Iterating over multiple arrays changes array values

If I iterate in bash over multiple arrays and printing its values then the values of the arrays changes. Why?

#!/bin/bash

a=("02" "20")
b=("02" "20")
n=("02" "20")
p=("02" "20")

for p in ${p[@]}
do
    for b in ${b[@]} 
    do
        for a in ${a[@]} 
        do 
           for n in ${n[@]} 
           do
             echo $b-$a-$n-$p
           done;
        done;
    done;
done
echo "${a[*]}"

The command echo "${a[*]}" yields 20 20 but should be 02 20

Thanks!