A numerically lower number does not necessarily mean that the course is a prerequisite to a numerically higher numbered course. Check the catalog for the actual prerequisite relationships between the courses.
http://guide.berkeley.edu/courses/ugba/
For example, UGBA 101A and 101B list prerequisites of Economics 1, Mathematics 1A or 16A, Statistics 21, or equivalents. They do not list UGBA 100 (or each other) as a prerequisite.