In ntp.conf(5), there is a wrong description:

 allan allan
         The argument becomes the new value for the minimum Allan
         intercept, which is a parameter of the PLL/FLL clock dis-
         cipline algorithm.  The value in log2 seconds defaults to
         7 (1024 s), which is also the lower limit.

Yes.  log2(1024) is not 7, but 10.  There is also the same description
in /usr/share/doc/ntp/miscopt.html, and they have been obsoleted.

=46rom ntp-4.2.0 (currently contributed in FreeBSD) and later,
this variable "allan" is set a real number (not an exponential number)
and defaults to 1500.

The default value is defined in src/contrib/ntp/ntpd/ntp_loopfilter.c.
The old version (ntp-4.1.2) was:

 #define CLOCK_ALLAN     10      /* min Allan intercept (log2 s) */

and the recent version (ntp-4.2.0 and later) is:

 #define CLOCK_ALLAN     1500.   /* compromise Allan intercept (s) */


The description has fixed in ntp-4.2.2:

 allan allan
         The argument becomes the new value for the Allan
         intercept, which is a parameter of the PLL/FLL clock
         discipline algorithm. The value is in seconds with
         default 1500 s, which is appropriate for most computer
         clocks.

Fix: Here is a patch for CURRENT.
How-To-Repeat: man ntp.conf