Right, the same test. I did get individual reports for my girls in the past few years, and I tracked the changes. The one for 2014 will be held next week.
I think most IS took the ISA. My son had it last year. His school compared his school average with the world average and "other like school in HK" average. A way to let parents know their result are above World and HK average.
Where are you getting data that VSA is out-performing other school substantially? It looks to be ahead in Math, but similar or behind the ESF system in reading and both writing. Are you getting the grades mixed up?
You can see overall ESF scores here. This is one year older than the VSA results published on the website but should still be comparable.
http://www.qbs.edu.hk/sites/qbs/files/QBS-ISA-Document-For-Parents.pdf