charging rates

In reviewing my supercharge bill I found that I was charged various rates at one particular charger in Brunswick, GA 6 min@ 0.28/min $1.68 2 min @ 0.83/min $1.66 8 min @ 0.52/min $4.16 It was also labeled at Tier 2, Tier 3 & Tier 4. I don't understand why this happens? Is it because I stayed...