NikonD800 question

why do Iget a 16-bit monochrome file when the camera shooting modus is set to 14-bit captureor do I?

The extra two bits are just padding (set to 0 or 1). The TIFF and PSD don't support intermediate values between 8 and 16 for color depth.

As far as a monochrome image, the three 14bit channels are being combined to give you a single channel. I haven't done the math to see it this can be expected to bully use the 16bits available in a TIFF or PSD channel.

If you use JPG it will all be compressed down to 8bits per channel.