I wouldn't be concerned. A quick scan of some of the threads of the last few days will give you an idea of what may be a factor behind this. Bucks' test has become very popular among out-of-catchment parents, some of whom live a long way away from Bucks schools. These are mainly self-selected children, i.e. opted in to the Bucks test, probably after a lot of tuition and practice, and probably entered for many different tests in their home and neighbouring regions. Being self-selected, those children are likely to score highly when compared to many Bucks children, the majority of whom haven't been tutored and haven't had any preparation beyond the practice test in their schools shortly before the real tests.
What all this means is that your result can't really be viewed in the context of any meaningful average because the mean has been significantly skewed. I've used a sporting analogy once already today, but here's another one:
It's a bit like turning up to your child's school sports day to find that in the 100m sprint, the neighbouring school's 10 fastest runners have been entered alongside your child's class of 30 students - all of the class. If your child would normally have come 15th in his class out of 30, he'll probably now come 25th out of 40. He hasn't become slower overnight, the race has just been loaded with pre-selected runners.
Edited to add: The kick in the teeth comes when at prizegiving, 5 of the visiting runners live too far away to come and collect their medals. Their places in the standings aren't automatically taken by the next 5 fastest children, but by the drawing of lots, partly influenced by how fast the Headteacher thinks each child can run and how many blisters they had on the day.