One of the biggest changes to the Achievement system in the transition from Xbox 360 to Xbox One is the ability for developers to add Achievements whenever they feel fit. Microsoft has already said that there will be restrictions in place to keep developers from breaking Gamerscore, but more information has come out regarding the positive intent of the system.
When speaking to Official Xbox Magazine, Xbox Live program manager Chad Gibson stated "the corollary is that with a lot of games today, three updates later it's a nice evolution of that game -- it's a different game that's been modified and adjusted, based on what people are enjoying and having fun with. And we think that Achievements should match that."
Gibson continued: "if a game's been on the market for a while, and they have the opportunity to add all these new Achievements -- if you start playing a game a year after it ships, there's probably 2000 Gamerscore to get for that game or more. And so, for the completionists, they're going to have more work to do."
Sadly, it's increasingly true that a game isn't necessarily "complete" when it releases, so it does make sense to allow some wiggle-room so that Achievements can evolve alongside it. An alternative would be for developers to actually have finished their games by the time they release. I'll quit with the crazy talk now.
Microsoft won't let developers "overwhelm" Xbox One games with post-release Achievements [Official Xbox Magazine]