Surprised nobody has responded to this thread yet,
I am not very experienced in electronics but I have a thought, the potentiometer (SVR9?) looks like it is a part of a relatively complex resistor divider network connected to that op-amp, by changing the two resistor values like you mentioned you would be changing the whole system, its not as simple as the tow resistors adding together. in other words, when you turn that pot you are not changing resistance you are changing voltage (as far as the op-amp is concerned), and by changing their values you affect the range of voltage that the pot can output to the op-amp.
That's my guess, They probably could have made it easier to adjust but ultimately they most likely wanted something simple and cost effective.