if you have a look into the source code you will find this:
public void addBox(float x, float y, float z, int w, int h, int d, float expansion)
x,y,z are the coordinates of the box
w,h,d the dimensions
and as you can see they are integer values this also applies for all other boxtypes e.g. shapeboxes, trapezoids...
it would be possible to change it to float, but all models then have to be changed too
There are actually lots of overloads of the addBox function. ModelRendererTurbo.java line 837 adds one that takes float parameters. Internally, it just converts this into a shapebox, rounding up the length of each edge for the starting box