This study explains mathematical difficulties of students who immigrated from the Former Soviet Union (FSU) vis-à-vis Israeli students, by identifying the existing bias factors in achievement tests. These factors are irrelevant to the mathematical knowledge being measured, and therefore threaten the test results. The bias factors were identified through a structured process that included analysis of mathematics achievement test results, as well as interviews with immigrant students and "culture experts." The prominent bias factors among the immigrants were primarily specific test items unfamiliar to their way of thinking as well as insufficient attention to item instructions that appeared in a different typographic presentation. This paper discusses the theoretical and practical implications of these findings.