Two immediate things come to mind here:

Capacitance accuracy - MOS Caps are nonlinear, suggest you go to a MIM type that exhibits more linearity. Sometimes this is also poorly modeled.

Current Accuracy - How is your current being defined? Unless there is an external current reference, or some trim/adjustment, most internally defined current sources have an absolute value error that is pretty large.

Two good places to start. After that, you need to start looking at the min-max accuracy/tolerances of all the parts and pieces.

If you want an accurate current (without trim) use an external reference resistor.

Bandgap drives a V to I converter, which uses an external reference resistance.

There are gobs of chips out there that have been doing this for years.