The problem is most likely due to high tolerance components. AKA acceptable margin of error.
Up to 20% on potentiometers, trimmers and capacitors.
Better capacitors can be 5-10% off and still acceptable. 20% is typical with potentiometers especially with the types we as DIY enthusiasts commonly use, many which are labeled but are knock-offs.
Those differences when combining more than one component (potentiometer, trimmer & capacitor) are going to add up creating a scenario where you will have to experiment with the final resistor value to achieve the desired effect.
Additionally you are supposed to use a C50K Pot. The anti-log taper potentiometer is used to ensure that controls have linear characteristics as perceived by the human ear. (This is not as important as tolerance).
Also try testing your components before installing them or vary the key resistor etc to get what you need.
A final possibility could be just a simple build error. Both of us have built many and I can only speak for myself when I say I still make errors.