In 14 months, there have been 3 major versions of Windows and at least 14 tuesdays patches, more than enough to disrupt the benchmark.
In addition, the overall rating doesn't mean much, you have to compare the different sections (processor, graphics, I/O), already it will tell you where the evolution is taking place.
If not, have you noticed an effective change in the operational behavior of the machine? The benchmark is a bit like just doing a 400DA to evaluate a motorcycle.