Why do 16 bit systems have a minimum dBFS of -96?
You have used \$\dfrac{1}{32767}\$ and this is the peak signal level. The peak to peak signal level is therefore 2 LSBp-p. But you can have a smaller signal: -

The smallest signal is half of this (i.e. 1 LSBp-p) hence another 6dB takes you down to -96dBFS