My guess is the agent queries WMI for the chassis type - what OS is installed does not (and should not) make a difference, it is the hardware that it attempts to classify.
Reference: http://henkhoogendoorn.blogspot.com/2014/04/chassis-types-and-query-based.html
I was told by Techaffinity (as the Discovery app isn’t a FW app) that:
“if the device does not have any value for the field "IMEI" then we will send the fields related to asset_type "Computer" and if the device has some value for the field "IMEI", then we will send the fields related to asset_type "Mobile".”
So what we do is run a report on our Assets and for the ones that need to be fixed, we do them one at a time or if its a lot, we take the data from the report and create a file to import to fix the fields that are wrong. This is not the best way, but it does allow us to keep it mostly cleaned up.