charging cost prior

I am confused by the prior and actual charging costs: For example:

I have 19 Charges at Location A.

Location A changed its price from 0,28 to 0,22 cent per kWh.

I go into my last charge on Location A, click on Location A > go to Price > enter the new 0,22 Cent price (added) than > save ... I get asked" Would you like the prior pricing to prior charges and the new pricing on future charges? > I say: Yes... and ALL prior pricing are changed to 0,22. Why? 
I do not understand this.