The UOMs shown in UI are highly related to the user preferences/profile and formulation spec preferences . It is hard to make a generic API to respect various of specific preferences. We don't include the logic to present the data as it in API. They are converted to its original value by UOM.
If you want integrate PLM with your application to present same as UI, I am afraid you have to maintain a UOM conversion map in your middle-ware layer.