Why does this bash script work even with the typo

So I have this bash script to find the Fibonacci sequence for n terms. Its definitely not great code, but I notices that it still works with a typo

read -p "Enter which term of fibo seq to find: " n

table=(1 1)

for i in $(seq 0 $(($n - 1)) )
do
    if [[ -z ${table[$i]} ]]; then
        a1=$(( $i - 1 ))
        a2=$(( $I - 2 ))
        val=$(( ${table[$a1]} + ${table[$a2]} ))

        table+=($val)
    fi
done

echo ${table[@]}

inside the if statement evaluating the expression for a2, there is a capital 'I' instead of 'i'. My understanding is that shellscript is case sensitive but I'm new to it. So can anyone explain why this works?

a2 will always be -2. Starting with bash 4.2, you can index into an array using negative indices, much like in Python. So with or without the typo, you are always accessing the second last element in the array.

The statement a2=$(( $I - 2 )) where $I is not declared is equal to a2=$(( - 2 )) which means a2 will always be equal to -2.