Penalty for early settlements

We had an account from Joshua Doore we were paying very well, but when we went to settle the account we were charged inconsistent amount . The one from the last statement was different than the one they have at the store, we had to pay extra because there were uncharged interests. And it was not our first time buying from the JDG and we thought we knew how the system worked but to our surprise it "changed". We took a vow not to set foot at JDG stores ever again.