In my case, I have done a lot of math work and emphasized it in the application.
Probably they thought you did it long time ago, or it wasn't sufficient, probably you should have made more emphasis on it, I don't know what might have happened. But they thought you were a strong candidate and admitted you (with that condition), compared to other people how might have had the math background they wanted but lacked other things.....
If I were you, I wouldn't think too much about it and just take the class in the summer anywhere to comply with it..... you might not be the only one....
Good luck!!