Why doesn't a resistor on the non-inverting lead of the op amp affect dc offset (non-inverting amplifier)?
If the source impedance(several kilos) of the dc offset voltage source is small compared with the load impedance of the non-inverting input(several megas) of the op amp, there will be no loading effect of the offset voltage. Typically this is true for your circuit, you can try increase the source impedance of your dc offset source, see if dc offset lowers.