Originally, the idea seems to have been that a student should take Key Skills at a level higher than their last achievement.  This, as any sentient teacher will immediately recognise, is an impossibility given the context of a student studying, for example, up to 5 AS level subjects - there are not enough hours in the working week to achieve this aim. As you may imagine, something had to give...

Without getting into a debate about what may have to give in the future, in the present the thing that gave was, effectively, two requirements:

Firstly, that students would necessarily progress to Key Skills at a higher level than their immediate past level of achievement and, secondly, that students would study for all six Key Skills.

The situation, at present, seems to be that individual institutions make decisions about the breadth and scope of their Key Skills provision (not a very helpful analysis for anyone looking for guidance about Key Skills provision and teaching but, hey, welcome to the wonderful world of decisive...err...indecision).