It often seems like there are some special seeds that are added by the developer. You can tell by the weird names/numbers that often have a double meaning. If they go to the effort of doing this, why do updates constantly break these seeds and force everyone to have to find new seeds if they want to use all the features in the new update. Seems so pointless and ruins a lot of the fun when there are older seeds that have been well explored and mapped out by others.
New updates add new features to the game like new ore blocks and stuff. Other times there may be some bugs and glitches with the world generation. To resolve the bugs or to add new blocks the seed generation algorithm must be modified. The bigger the change is the more likely it is going to change how worlds are generated.