Ok, I made a spreadsheet and I see where you're coming from.
With your method, by definition, a voltage input of 1.67v would give an ADC conversion of 167. The voltage calculated from that would then be 1.67v.
If I were to do it my way, I would scale the voltage divider so that at a 5v input to the voltage divider, I'd get a 3.0v input to the ADC (using a 3.3v reference). A 1.67v input to the divider would put 1.00v at the input to the PIC. With a reference of 3.3v, that would come out to a conversion of 310. After converting back to the original voltage, I would get 1.66v instead of 1.67v.