There are no differences. Classes are offered in different time slots for convenience. However, class contents differ between grammar retake classes and seminar retake classes. Therefore, if you want to retake grammar classes, take German IA (grammar) and IB (grammar). If you want to retake seminar classes, take Primary German A (seminar) and B (seminar). Note that you cannot carry credit in IB class to IA class, and vice versa.