Offset Error Changing after Calibration of Output
The INA145 has an input offset of max +/-1mV:



But this offset is the due to manufacturing process and it remains as is, therefore the calibration shall eliminate this issue. Next is the drift, which is measured in few micro volts per celsius, multiplying this with gain you get some very small error. 

IMO you do use some resistors that have high TCR, they are not suitable for precision voltage dividers.