There have to be available studying slots in the higher study semester, which is seldomly the case. If you fulfil regular admission requirements and you have completed the necessary creditable courses at time of application you can apply for a higher study semester. For example, if you want to apply for the 2nd semester in summer, you need 30 completed ECTS from the Statistics Master program at time of application (usually in January) in addition to the credits needed for regular admission.