Whenever DB2 "truncates" a log file, there may be a relatively large
apparent log gap shown for this HADR snapshot monitor element. This is
mentioned in HADR documentation, for example here:
http://publib.boulder.ibm.com/infoce...c/r0011478.htm

What happens is that DB2 effectively jumps over a range of unused LSNs
(log sequence numbers), which fools HADR into thinking the distance
between the primary and standby is greater than it really is. The
extra gap vanishes once the standby moves to the next log file; that
occurs once there is some operation on the primary subsequent to the
operation that cause the log file truncation. Online backup is a
typical "culprit"; the ARCHIVE LOG command also has this effect. The
artificial log gap may trigger a related health monitor alert as well.

We anticipate making some changes in a future release to help with log
gap accuracy in the presence of a truncated log file.

Regards,
- Steve P.
--
Steve Pearson, IBM DB2 for Linux, UNIX, and Windows, IBM Software Group
DB2 "Portland" Development Team, IBM Beaverton Lab, Beaverton, OR, USA