DNG files are usually 16 bit files or more precisely 48 bit composites of R,G & B. So whilst your original may be 14 bit it is wrapped in a 16 bit container with greater headroom because there is no need for the jpeg which the camera displays on the LCD screen.
Okay, I get that part (that the DNG is up-converted to 16 bits per channel) but if that's the case, then why isn't the range of exposure-stop adjustments -8 to +8 (ie 16-bits of adjustment) instead of being -5 to +5??
I guess most photographers needing that degree of latitude would add a big stopper filter to the lens??
According to the Canon website (Canon U.S.A. : Support & Drivers : EOS 5D Mark II), my EOS 5D Mark II produces a 14-bit RAW file, yet when I bring it into LR5 (as a .dng), the exposure slider goes only from -5 to +5 (10 stops); why doesn't it go from -7 (or -8) to +7 (+8)?
Bit depth has nothing to do with dynamic range assuming that's what you're referring to with the +/- 'stops'.
There are very few true 16-bit capture devices that we can afford. Adobe considers anything higher than 8-bits per color to simply be referred to as "16-bit" even if they are not. It's pointless to show 12 vs. 14 bit and so on, the real factor is, do we have more than 8-bits per color to edit with?**
In fact, even Photoshop doesn't use 16-bit but rather 15+1 bit, has since day one, LR is probably the same, not that it matters.