Simone Biles nailed her beam routine in Tuesday’s final, earning an execution score more than three-tenths higher than she had in the qualifying round, when she stumbled backward on her dismount. But her total score, 14.0, was slightly lower.

That’s because she changed a single skill: the dismount.

In gymnastics, each skill is assigned a letter value that represents its difficulty. Skills rated A are easiest, while more difficult skills are rated sequentially using letters of the alphabet: B, C, D and so forth (and yes, they’re in reverse of the letter grades you wanted to earn in high school).

During the qualifying round, Biles dismounted with a full-twisting double back, which is rated G. In the final, she dismounted with a double pike, which is rated E, so two letter values easier.

Each successive letter is worth an extra tenth of a point: An A skill is worth 0.1, a B skill is worth 0.2, and so on. That means in terms of absolute difficulty, doing the easier dismount cost Biles only two-tenths. But because the total difficulty score for a routine is based both on the individual skills and on bonuses for linking multiple skills, changing one move can have a snowball effect.

The Code of Points, which governs scoring in gymnastics, awards a two-tenth bonus for connecting a B skill to an F (or higher) dismount. Biles normally receives that bonus because she does two back handsprings, each rated B, into her G-rated full-twisting double back dismount. But by downgrading to an E-rated dismount, she lost the bonus.

That meant her difficulty score went down by four-tenths of a point, compared with the routine she used in the qualifying round: 6.1 instead of 6.5. That outweighed her improved execution.

Still, balance beam finals are quite unpredictable. Several other gymnasts made mistakes, and Biles, with her less difficult but better-executed routine, won a bronze medal even after finishing sixth in the qualifying round.