Does this make since? Went in ffor car loan with bad credit and co-signer, walked out with no car!?

Went in today to a dealership (carmax) thinking that my family will finally have a larger car, since we are expecting our second child. We do not have the best credit because of past mistakes but are working on it. We have a co-signer with great credit, so we thought we had it. Well, we were told that my co-signer qualified just fine for us to get the car but our credit was bad, that it why we couldn't get it. Duh! Isn't that the purpose of a co-signer? Then they tell us that our score is 115 points lower than what we know it is (we pay monthly for our credit score from the 3 leading places). Does this make any since at all? I have never heard of having a co-signer with great credit, but because that person didn't have good credit, they couldn't get a car. I am very confused and angry!