Bug 93201

The manual page vmstat(8) correctly describes the -n option, that it shows 2 disks by default:

     -n      Change the maximum number of disks to display from the default of
             2.

but other paragraphs states it shows 3 disks by default (emphasized by ***):

             Any number of -p arguments may be specified on the command line.
             All -p arguments are ORed together to form a matching expression
             against which all devices in the system are compared.  Any device
             that fully matches any -p argument will be included in the vmstat
             output, up to ***three*** devices, or the maximum number of devices
             specified by the user.

[...]

     disks   Disk operations per second (this field is system dependent).
             Typically paging will be split across the available drives.  The
             header of the field is the first two characters of the disk name
             and the unit number.  If more than ***three*** disk drives are config-
             ured in the system, vmstat displays only the first ***three*** drives,
             unless the user specifies the -n argument to increase the number
             of drives displayed.  This will probably cause the display to
             exceed 80 columns, however.  To force vmstat to display specific
             drives, their names may be supplied on the command line.  The
             vmstat utility defaults to show disks first, and then various
             other random devices in the system to add up to ***three*** devices, if
             there are that many devices in the system.  If devices are speci-
             fied on the command line, or if a device type matching pattern is
             specified (see above), vmstat will only display the given devices
             or the devices matching the pattern, and will not randomly select
             other devices in the system.

Looks like one day the the default disks shown changed from 3 to 2, but only the  -n option has been corrected, as this diff shows: http://www.freebsd.org/cgi/cvsweb.cgi/src/usr.bin/vmstat/vmstat.8.diff?r1=1.10&r2=1.11

Fix: 

Change the four emphasized words "three" in the full desription to "two".
How-To-Repeat: See vmstat(8) man page.