Vocabulary levels get longer the higher the deck level. Level length is determined by the available number of words at that level. For example, if level 1 only has 10 words and level 5 has 50 words, it may take 5x the effort to reach level 6 as it did level 2. The idea is to enable users enough opportunity to practice newly introduced words sufficiently before prematurely gaining a new level where even more new words are introduced. If this does happen, a few incorrect answers will bring the user back down to a level where practice is comfortable.