For the Bayley-III, 1 standard deviation (SD) below the mean and above the mean correlate with the following: Scaled score range 7-13 (3 points below and above the mean of 10) and composite score range 85-115 (15 points below and above the mean of 100). 1 standard deviation also correlates with percentile rank equivalents of 16 (1 SD below the mean) and 84 (1 SD above the mean). There are pictures of bell curves online that display all of these score correlations, as they are standard psychometrics, not unique to the Bayley. The picture might help you visualize the correlations.
Relative to the practice test question, a scaled score of 9 is well within 1 SD of the mean (although below the mean of 10) so is classified as “average.” The Bayley-III manual classifies scores of 8 or greater as “average” as they are within 1 SD of the mean. Scaled scores of 6 and 7 are low average (or often interpreted clinically as borderline) while scores below 6 are “below average.”