It's not intended to be norm referencing, but a starting point for fitting the candidate's work into the marking grid. It's not how the final mark is assessed. It's not done in "grades" but raw marks. A low "B" is the average (median) for languages.
This is in the public domain in the examiner's reports, so no big secret. Assessors have a very clear idea what a piece of work scoring 10/15 for communication and content looks like. If it's better than a "10", assessors will then start looking in the top boxes to see where the best fit is; if it's worse than "10", the lower boxes are looked at. It's just a way of saving time, so that the assessor doesn't have to go through all the boxes. It also sets a standard. We also have about 20 pieces of officially graded work, against which we can compare candidates. It's actually criterion referencing. Approximately 25-30% of marking is double-checked by a senior assessor.
Assessors have nothing to do with grade boundaries and don't even know for sure that a piece of work will be awarded a certain grade. The work is marked against set criteria and the grade boundaries are set after all the raw marks have been collated. It's at this stage that political shenanigans play their part.
ONS, your dd achieved a "B" in writing, so I would hope that she can string a sentence together, but lower achieving candidates can't - and I did write "unaided". Anecdotal, I know, but...my own ds has just achieved As in Year 10 in maths, history and geography and is predicted more As next year in most GCSEs, including Latin. His lowest predicted grade is in German (A) and it will be the controlled assessments which will let him down. I refuse to do them for him, although he obviously has all the resources at home he could need. I know for a fact that he can't formulate from scratch a complex sentence with all the niceties of adverbial phrases and idioms, etc. of the type needed for an A*. I didn't intend to insult your dd. Sorry!
When GCSEs were first introduced, writing wasn't a compulsory component. It's the most difficult skill and, arguably, the least valuable. My personal opinion is that it's a waste of time for the weakest candidates and that they should concentrate on reading, listening and speaking, which are likely to be of more use in life. Even before coursework and controlled assessment, grades for writing were usually worse than they were for the other skills. Writing remains the skill which differentiates amongst the best candidates. For a piece of written work to be awarded in the top band, it has to be something special.