Chosing a package is according to different matric?
0402 means it is 40mils x 20 mils (0.04" x 0.02") which is essentially 1mm x 0.5mm. 

It can't be exactly both, obviously, so chances are it will be 'hard' metric and the inch dimensions are approximate.  

Looking, for example, at this datasheet we see that these particular Johanson RF inductors are hard metric: