I didn't think I knew this, but I got it right! When I saw the possible answers, I realized I'd seen this symptom before and was able to deduce the correct answer.

I was working on a customer's databases with which I was unfamiliar. They had a two servers, a primary and secondary (the app server handled failover). The primary was RTM while the secondary had service packs. Not only that, but the server net libraries on the primary were later service pack versions! I applied service packs, but had been left wondering how that could have happened. Now I know what might have happened to cause the discrepancy!