The students in the US are positioned in the middle among the nations on the international tests of academic achievement. This moderate achievement is a matter of great concern as it reflects the output of the US education system, as well as the input, in terms of human resource, available to the market. One way of probing this moderate achievement is its comparison with the achievement of students from the top-performing countries. Much of this comparative research lacks a fully-developed, systematic, and theoretical explanation of causes, notably ignoring the influence of culture on achievement. It is within this context that the present study was conducted. Taking a relativist position, I compared the middle school students in the US with those in the top-performing countries in East Asia by engaging a theoretical framework comprising motivation, self-concept, and culture. I used Eccles and Wigfield's model of expectancy-value theory of achievement motivation (Eccles, 1983, Eccles & Wigfield, 2002), Marsh's internal/external frame of reference for self-concept (1986), and Triandis' (1995) individualism/collectivism constructs of culture to find an explanation of the identified achievement gap. I used a subset of TIMSS (Trends in International Mathematics and Science Study) 2015 data of 36,115 middle school students in Chinese Taipei (Taiwan), Hong Kong, Japan, Korea, Singapore, and the US. One-Way ANOVA test and structural equation modeling were used to look into peer achievement, science achievement, math self-concept, intrinsic value and utility value of math and their influence on math achievement of students from two cultural groups. The results inform that in the US, peer achievement links differently to self-concept and intrinsic value; self-concept and utility value have high mean values; and they associate to achievement weakly or negatively. Thus, middle school students in the US do not believe in competition with peers; their perceptions about themselves and about the usefulness of the domain are inflated; and who value math more, score low on math achievement test. These student characteristics contribute to the moderate math achievement of students in the US.