I don’t have an answer for you but it reminded me of a quote I heard on a podcast that I can’t find (I think it was RadioLab). If you have one thermometer you know the temperature. If you have two thermometers you have confusion.
How far apart are the readings? How far apart are the thermometers?
In my son’s room, through pure accident, we have three thermometers scattered around the room (none are smart or report anything to openHAB but all three do have a display so I can see their readings). It is not unusual for them to be up to 3 degrees C different from each other. Part of that is caused by the fact that those different parts of the room could very well be different temps. But a lot is also that measuring temperature is just not that precise and usually close is good enough.
Looking at the Z-Wave database, it seems like you have a calibration parameter. You should be able to set it in HABmin to -40 less than its current value to have the sensor agree with your manual thermometer, assuming the manual thermometer is your definition of accuracy. Not that it will take some time for the parameter change to take effect.
<Label lang="en">Temperature Calibration</Label>
<Help lang="en"><![CDATA[ The range of calibration value is -100 to 100 (or -10.0 to +10.0 degrees).<br>
The value contains one decimal point. E.g. if the value is set to 20, the<br>
calibration value is 2.0 ? (EU/AU version) or 2.0 ? (US version).<br>
The calibration value = standard value - measure value.<br>
Example: if measured value = 25.3? and the standard value = 23.2?, then<br>
calibration value= 23.2? - 25.3? = -2.1? (0xEB).]]></Help>