Sorry Sillygoose but I disagree with you!
No - not a built in safety margin in the AQS, but due to people above you choosing to go elsewhere. And, as you say, you cannot be sure of what people ahead of you are going to do - this applies to both rank and score, so I am not really sure why you think score is a better measure, when it does not relate across various years. Rank does. And, as AQS's etc have not been finalised as yet, previous year's rankings are the only thing current years applicants have to go on. A person who is ranked 10th last year can be compared with a person that ranks 10th this year - they are in the same position! However, their scores may vary massively - the year before last, due to the weighting, the scores were in the 300s - the last two years they have been in the 200s - but the rankings remain comparable!
Scores fluctuate widely depending on the difficulty of the exam and, due to standardisation, how many people took the exam. The rank tells you exactly what position your child is in. As you know how many places are available across the county and in various schools in particular, you can have a pretty good guess as to how likely you are to get a place in a particular school.
The reality is, the score/rank letter received makes it very clear that, regardless of how well your child has done, there is no guarantee of a place in a Grammar School. Although it is "likely" that anyone scoring the AQS or above, who has listed the GS on their CAF, and lives in the appropriate area will get offered a place in March, it is misleading to say that they are "guaranteed a place".