Two things come to my mind:
- WaniKani Content Additions: maybe you are seeing a few of these new items
- you recently gurud (<- how do people say this?
) some old kanji which unlocked the old vocab items => you don’t need to guru all kanji per level for the level up. Therefore, technically it is possible, in case some item is giving you a hard time, you’ll guru it after you already had moved on some level(s)
Getting old vocab from the previous level always can happen since some of the items will not be unlocked before the level up. Getting items from older levels (before the previous one) could be the result of one of the reasons I mentioned.