This might likely be a vestige of the episode originally being produced as a short.

The episode 'Water Park Prank' is not paired with any episode despite being an 11-minute segment.

However, the order episodes go in pairing order when in comparison to the airing/package/streaming order differs, thus the paired order is shown on the right side. The show's first six seasons were packaged into pairs for broadcast. The show would air for ten seasons and 283 episodes. The show premiered on Cartoon Network in March 2010 as a sneak peek and would officially premiere the following month.

The following is a list of episodes of Adventure Time.

Still, he persists in trying to steal her away, and Finn and Jake, along with Lady Rainicorn (a cross between a unicorn and a rainbow) do their best to keep her from harm. He's decided he should wed Princess Bubblegum, though she doesn't want to marry him. Usually that evil comes in the form of the Ice King, who is in search of a wife. Assisted by his magical dog, Jake, Finn roams the Land of Ooo righting wrongs and battling evil. I was confused when they listed Season 7 as having only 25 episodes and that Season 9 started with Orb its like they just jumped ahead and skipped Season.

Twelve-year-old Finn battles evil in the Land of Ooo.

Adventure time season 9 episode 19