These are both 1962 Chevrolet cars.Comparing a Walker class to a Miranda class is like comparing artwork from an Impressionist to artwork from a Cubist. Two different styles, from two different visual continuities.

If you can accept that both these cars can be made in the same year by the same manufacturer, than accepting that Starfleet can make both the Miranda and Walker class within several decades of eachother, isn't that difficult.