Generating a random sequence of 6 numbers with Python sometimes yields a result with only 5

I have a small problem with a function that should potentially generate a random 6-digit code. The code in itself is absolutely straightforward, using python 3 I do it like this:

def secret_code(num=6):
numbers = string.digits
code = ''
for _ in range(num):
    code += random.choice(numbers)

return int(code)

Now, there is a gazillion way to do this but I am not questioning the validity of this method, in particular, against others, my issue is that sometimes this function returns a 5-digit code.

I have tried a for loop with 1000 cycles to test how often this happens, in this way:

for _ in range(1000):
    code = secret_code() # calling the function above
    if len(code) < 6:
        count += 1

ratio = count/1000
print(ratio*100) # at this point the test has given back 0% all the times

and it always comes back 0% of the time.

However, when applied to a website, for example, I am using it to generate random verification codes to send to new users, sometimes (I don't mean 50% of the times, of course, but it's not even 0) it comes out with 5 digits instead of 6 and, for the life of me, I can't figure out why.

Does somebody have any idea why this occurs? and more importantly, why doesn't it show up in the "for loop test"?

Thanks