Couple of possible reasons!
1. Pre-release codes: Sometimes reviewers will get a code to redeem/play early so they can have a review ready on launch day. If the achievements are already uploaded, it is possible to earn achievements during this period. I've seen it with DLC as well.
2. Region: Sometimes games in multiple regions have different achievement lists. A lot of time they share one. If the game released early somewhere else and it shares a list, that is a possibility. Probably a pretty rare instance with DLC though.
3. Cheating: Unfortunately, it is always a possibility. This community is pretty good at removing cheaters. If you have suspicions about someone, follow this advice:
"Users on Exophase can assist with identifying cheaters by reporting them. In the header of each profile page there is a Report link that you can use. Please only use this for reporting cheaters, not for general issues."
I don't work/represent the site, but i am curious - what game are we talking about? Might help get a clearer answer!