Why can AS level marks only be carried forward within a 13 month period?

Cambridge does not offer a modular approach to assessment. The expectation is that candidates taking an A Level will complete their course within a two-year period. However, we do allow a ‘staged assessment’ approach to the two year course of study, which means that a candidate can complete an AS Level in one series/year and carry forward that whole result to be added to the result for the rest of the A Level within 13 months. 

In many cases, the A Level papers assume prior knowledge of the AS content. If we were to allow a longer gap than 13 months between the series in which the AS result was obtained, and the series in which the full A Level was obtained, this would not be in the candidate’s interest as there would have been a longer period of time elapsed since they studied and were assessed on the AS content.