[ElaineM]

I think the difference is because you ate food and took your vitamins just before the second test.
The same thing happens when we take tests which include blood sugar (glucose) and cholesterol. Fasting results are lower than non fasting results.
The vitamin D test I get every 6 months breaks the results down into how much vitamin D is from supplements and how much is naturally circulating in my body. Then the two are added together for a grand total.