The formula from the product manual is very slightly off from what the chip manufacturer has provided in their data sheet. It is to compensate for some of the extra circuitry that we have included on the board. However at the same analog output value, the compensation adds only ~1%RH.
One situation where you can get such a low reading is from the humidity sensor not being powered closely enough to 5V. This can be due to long wires connecting the 1125 to the 1018, long USB cable, or even the USB hub on your computer provides less than 5V. All of these combined can dramatically change your sensor output. Use a multimeter to check the supply voltage on the 1125.
If you have an external powered USB hub, try attaching everything onto that and see if the problem persists.