Your weatherstrip passes though several different mills on its way to you. There is a limited demand for this product nowadays, yet it still has to be manufactured in huge batches. First the spring bronze alloy is made, then it is sent to a rolling mill, then to a slitting mill, then to a forming mill.
The alloy is only made every couple of years. So every couple of years, the weatherstrip may change to look slightly more coppery or slightly more brassy. The different colors of weatherstrip perform the same, and because they are not lacquered, they will tarnish to a brown color.
We do not try to stock different finishes, but will try to ship an order with all the same color if there is a transition.