I wonder if anyone can answer a simple? question for me?

We have an old piece of tartan blanket, heavy and thick. The colours are different on each side, with only the white stripe (I think) in the same place.
My question;
How is such a tartan woven, so one side looks primarily brown, and the other side primarily red!?

This might be common on tartans, I don't know! If so please excuse my ignorance!!
Richard.