Incorrect xdpi and ydpi values for Some Motorola Devices

I am trying query xdpi and ydpi value of Motorola Droid2 global and Motorola Atrix to calculate screen width, These are the values i am getting

Atrix: Resolution: 540*960 xdpi: 160 ydpi: 160 calculated screen width is coming to 6.1, whereas originial value is 4

Droid 2: Resolution: 480*854 xdpi: 144 ydpi: 144 calculated screen widht is coming to 6.9, whereas original value is 4.

Same logic of width calculation works fine with Xoom, HTC, samusng phones and some other android devices. In those cases xdpi and ydpi reported are around 240

Are values 144 and 160 correct values?

If this is not the right approach any idea, how can we calculate screen width programatically?