Larson gets Rookie of the Year in a mildly controversial decision. Dude was driving for Arrow McLaren, had Tony Kanaan as a driving coach. Did very well in practice and qualifying, making it to the Top Six. Led during the race during a pit cycle. But he made a couple mistakes, one that almost caused an accident when he goofed on gear selection and speeding on entry into the pits (he isn't the first or last person to do that). Had an 18th finish.
Christian Rasmussen had a much slower month of May. A rookie to Indycar and the 500, Rasmussen (23 years old), raced a hard race, making some quality passes. Didn't screw up, but because he was racing for Ed Carpenter Racing, who isn't flushed with money, finished 12th. Teammate Rinus Veekay finished 9th, indicating a 12th place finish was close to what it could do best. IE, his finishing positioning wasn't a fluke of luck or timing. Arguably the best Rookie of this year's Indy 500.
The decision is made sillier, because Larson isn't going to care about this accolade, where as it could be a stepping stone for Rasmussen. Back near 20 years ago Alonso would get rookie of the year in a similar circumstance, except the top rookie was Ed Jones, who finished in 3rd. Certainly, with his performance at Indy, Larson will be expected back next year, a little wiser. But of course at Indy, the luck component can not be overlooked, ask Marcus Ericson or Parnelli Jones or the entire Andretti family. Winning Indy doesn't usually meaning being the most dominant... it just means you were the best of the survivors.