The code may be pretty bad, and it's interesting to see whether a candidate spots that it's bad (if they don't you're presumably going to get more poor code from them if hired) and what about it in particular they point out. It's an opportunity for them to show you what they know, rather than a box checking exercise where you know the answers.

I've actually done an exercise like this about a year ago, in Java, I remember I really cared about iterators at the time, and so even though it's probably not the worst thing wrong with the code I just kept coming back to these C-style for loops which I thought were reprehensible. I should ask my interviewer (whose department I now work in) what their impression was as a result of this obsession.

They even went one further and explained the answer they're looking for:

You lose data due to overflows which is what we expect

Seems like a reasonable question, though without more context that may or may not be true.