use calculation as the condition in loop

Question

I've a script named t.sh as below:

#!/bin/bash

num=2
for i in {0..$((num-1))}; do
        echo 'aaa'
done

for i in {0..1}; do
        echo 'bbb'
done

The output of ./t.sh:

aaa
bbb
bbb

I can't understand at all. $((num-1)) equals to 1, so I think aaa should be printed twice, just like bbb, why it shows only once?