Ostensible discrepancy between 'shred' and its documentation.
(shred (GNU coreutils) 8.23 on Xubuntu 14.10)
On the 'shred' info page:
‘-NUMBER’
‘-n NUMBER’
‘--iterations=NUMBER’
     By default, ‘shred’ uses 3 passes of overwrite.  You can reduce
     this to save time, or increase it if you think it’s appropriate.
     After 25 passes all of the internal overwrite patterns will have
     been used at least once.

If I try the '-NUMBER' method to specify number of iterations:

$ shred -1 file
shred: invalid option -- '1'
Try 'shred --help' for more information.
The other two methods for specifying number of iterations work.