For all you know you're seeing the HDD cache and not any kind of filesystem caching. evmar mentions SSD making a difference for workloads that should fit in RAM, which means HDD caching also would, for a very modest workload.
I know it's not the HDD cache. I can monitor physical disk requests vs logical filesystem requests (diskmon vs procmon). But I repeated it anyway on my SSD, and the results are the same.
