How can the you get two different results for the same test from the same blood draw?

I recently had some blood work done -- orders from two different docs. I did both orders at once to cut down on the sticks. Both docs ordered a specific test (A1C), but the results delivered to the offices were different. Not by a lot, but they were different. How is that possible? It wasn't just the same lab on the same day, it was off a single blood draw. Is that something I need to be concerned about reliability-wise?