Sam's Club bad gas?

[Yangs]

I'm one of the Sam's Club Member I been using their gas for the last pass 30 days. This morning while I was filling up with gas at Sam's Club I notice a lack of MPG on the highway but funny thing is that I didn't notice any poor performance. Every time I fill up full, I always reset the odometer just to keep track of my mileage of every fill up. I normally fill up when it gets to 380 miles mark with name brand gas such as BP and Shell. After filling up with gas at Sam's Club, my first gas bar disappear right at 60 miles on HWY but with other name brand gas such as BP and Shell I get up to 80+ mile until the first bar gas disappear city or HWY. I didn't understand why. Then I wanted for 120 mile mark because that's normally where the second bar disappear. But then it disappear right at 80 miles. I'm not sure if their gas have any thing to do with the poor gas mileage. If that was the case I will never purchase their gas again.

I change the plugs and oil before I started using Sam's Club gas. I have no modification done to the engine nor computer. My car is a 09 4 door Yaris.