Balance / balanced is one of those ideas that everyone seems to understand, and the word can conjure up an image: maybe of a seesaw, or a pair of weighing scales. In one sense, it's obvious how these work, but that's only because we have an understanding of Newton's gravitational theory; this understanding may be intuitive or conceptually-based.
However, this is not the sort of balance that we mean when we speak of a "broad and balanced curriculum" as the 2002 Education Act does in England.
The idea of a balanced diet is nearer to the 2002 use of balance because understanding the idea of a balanced diet requires a theory of nutrition that allows you to know what to include in a diet, and in what proportion. Without such a theory there's a risk that you will consume a range of things that will not do you any good. It seems that a lot of us do this nowadays.
And thus it is that, without a theory of curriculum, there's a risk that you will study and learn things that will not do you much good. Thus, when the Head of Ofsted talks about 'deep' and 'rich' but does not mention 'balance', alarm bells should ring.
I have written to Ofsted and to the DfE to ask what's going on.