Vitamin D (again) reduces cancer risk

It's starting to get tedious. In a recent article in the American Journal of Clinical Nutrition, Vitamin D (this time with calcium) reduces the risk of cancer (in a group of >55 year old Nebraskans) over a 4 year period by 76% (95% confidence interval 40-91%). At least they gave a useful amount of vitamin D3 (1100iu/day). Interestingly, even calcium (1500mg/day) alone showed a substantial lowering of risk, but it wasn't significant.
Now, the important thing to appreciate is that this study was only 4 years and that your typical cancer will have been growing for several years before it is diagnosed. So, is this a reduction in the incidence of cancers (less cancers starting), slowing of growth (thus fewer showing up over the study), or stopping the cancers that had already begun? Frankly, with such a dramatic improvement, it could be all three.