The showers I have used that have two knobs have something akin to a globe valve for each line
As you unscrew the handle, there is more opening between the plug and the body, letting more water through. The resistance of valves like this is quite variable when the plug is near the body. Once the plug is withdrawn a certain amount the resistance is small. If you were happy with the temperature that came from maximum flow through each of the hot and cold lines, it would be almost constant. Unfortunately, you probably want a temperature that uses all of one pipe you can get and just a little of the other. For me, I open the hot all the way and use the amount of cold to get the proper temperature. Changes in the geometry of the cold valve can change the flow significantly, leading to a change in temperature. In my shower, the water takes a noticeable step down in temperature after a certain time of use. The two valves are close together and connected by copper pipe. I take the change to be caused by the body of the cold valve heating due to conduction from the hot side. That increases the opening and necessitates an adjustment to the valve.