Some things are completely defined by the seed, for example location of biomes, location of crypts, etc.

However, the areas are only fully fleshed out when your character gets near them and that's where you get some randomness. For example, try saving the game, entering a crypt, reloading and entering again. The layout will most likely be different because the interior of the crypt is generated when you get in.

This is why the devs were telling people that as long as they had not entered a Mistland area before the update their map should be good to continue. This is also why online seed generators cannot show everything, like location of silver veins for example.