The 11+ exam is 'norm-referenced'. This means that children are ranked against each other and not by an absolute list of pass/fail criteria. The reason for this lies in the fact that it is essentially a competition. Suppose you had a tick list of 100 questions and everyone got all of them right. Under the opposite system, known as 'criterion referencing', everyone would then pass the exam and if a school only had 50 places you would have 50 children passing with the top score but not getting a place. Equally, maybe no one would pass the exam and then you would have an empty school. So the standardisation process plots children's actual scores against each other for every school they have applied for and the 'pass mark' is actually a fairly arbitrary figure which will vary each year depending on the spread of marks and the number of children applying. The score you are given is not an 'actual score' (say, 106/185) but a standardised one, which may say something like '128'.
An additional feature of the 11+ is age standardisation which is designed to remove the relative advantages of birth month when children are essentially being pitted against each other in a competitive exam. So a child born in September will be ranked against a cohort of other September children only, and so on, before some whizzy formula is applied to the entire cohort to ensure that the 'top' children get a qualifying score, regardless of their age within the cohort.
And finally, each of the grammar schools in Gloucestershire sets its own score. There is a heirarchy which you probably already know about, so one school sort of sits at the top and all the others take their cue from this one. This school calls the shots really as many parents consider it the most desirable, so it is able to use market forces to demand the highest scores. There is no coherence between scores given out by the different schools as they use different scales - for example Crypt uses numbers in the 100s and STRS the 200s. It is a bit of a learning curve for parents to be honest but you are on the right forum for any specific questions you have.
Does that make sense?