I fooled myself over this once (well twice!), so here is a possible scenario:
If you had several adverts running in the period you are reporting on, but deleted them because they were not what you wanted, Google will report relatively accurately the percentage time the remaining advert was served. So if we assume advert A was served 50% and B 50%, but you deleted B, you will see A having been served 50%.
If you are certain that you are looking at a period when only one advert existed then you have identified a bug, almost certainly in the reporting side.
I think a number of replies are failing to see that one advert, even served once, if it is the only advert that was served in that ad group in that reporting period, is served 100% of the time