The problem here is that pixels are just that, a single pixel on the screen regardless of dimensions, whereas Point, Centimeter or Inches are actual measurements.
As per the FileMaker docs:
FileMaker Pro now manages layouts using points instead of pixels. On most computer screens, 1 point equals 1 pixel.
On some high-resolution displays (such as the Retina display on iPhone), FileMaker Pro manages the display conversion to ensure proper screen rendering.
Moving windows to a specific position is usually going to be based on the current users screen resolution which is measured in pixels, so it makes sense to have these values as pixels. Also functions such as Get(ScreenWidth) will return the figure in pixels, although not tried on an device with a retina display...
Just consider 1pt to be 1px and hopefully it should all fall into place I guess.