In the United States, there is a summer gas and a winter gas essentially. If you want to read more about that you can here but the tldr is that the summer blend prioritizes less emissions and better mileage as people are driving more in the summer while the winter blend prioritizes better cold starting. I'm guessing it's just easier and cheaper to meet TopTier standards with the blend that GetGo purchases in the summer and too expensive in the winter. I definitely would prefer TopTier gas any day though since it's been shown to be better in the long run.