Answer:
In the Wordwide mode [16 bit], for each SLWR strobe that the external master does, a word is sent on the 16 bit FIFO bus. So if the host sends 17 bytes, it will be in sets of 8 words + 1 byte and the 9th strobe of SLWR [asynchronous mode] will have the unwanted data byte [FD 8:15] which can be discarded.