as per free_electron's comments on input bias current. i've made some calculation for the effect of Voffset, Ioffset and Ibias. my finding is that this is not AC related error?, its in the form of voltage offset error, which i think can be "easily biased"? from the formula below, assuming opamp's input impedance infinity (ignoring opamp input impedance, it seem doesnt matter when looking at 1Kohm source from 1Mohm input opamp), and assuming circuit is working in nonreactive region of Zs (external resistors), the plot in second picture, shows this offset error. though there's 1200% error at low voltage, but its not as bad as it sounds i think :-|

summary from the formula:

-Voffset will be affected (exagerated) by opamp (closed loop) gain.

-error generated by Ioffset at noninverting input will be affected by opamp gain, and source impedance (including the divider)

-error generated by Ioffset at inverting input will be affected by feedback resistor.

-they all accumulated to generate DC voltage offset. which i believe can be corrected with voltage bias somewhere.

-since In error is negative term, it can be made to cancel the other 2 positive error terms, ie caused by Vio and Ip

-POM = point of measurement. loading effect is also studied, but... at nonreactive region only

FWIW. i stand to be corrected.