Odd occurrence with log files - one is missing, the one after it is double size

[Toby]

In log4j.properties I have these lines:

log4j.appender.union=org.apache.log4j.RollingFileAppender
log4j.appender.union.File=log.txt
log4j.appender.union.MaxBackupIndex=9
log4j.appender.union.MaxFileSize=5MB
.... (etc)

Today I noticed that this was my log files:
 
1496540 Jul 13 10:35 log.txt   (current log being written, so smaller)
  5243118 Jul 13 09:37 log.txt.1 . . . . 5 MB
  5242959 Jul 13 04:17 log.txt.2. . . . 5 MB
  5243002 Jul 12 21:40 log.txt.3. . . . 5 MB
  5242915 Jul 12 18:35 log.txt.4. . . . 5 MB
  5243122 Jul 12 17:29 log.txt.5. . . . 5 MB
 10486224 Jul 12 16:33 log.txt.6. . . . 10 MB
(No log.txt.7)
  5243101 Jul 12 12:30 log.txt.8. . . . 5 MB
 5242980 Jul 12 11:07 log.txt.9. . . . 5 MB

It appears that for what are now files 6 and (would-be) 7,
it *thought* it had switched files, but somehow didn't actually
close one off and start the next.

Has anyone ever seen anything like that?  --- In nearly 7 years of running Union, I never did.
Or have any idea what happened?

There was one Union Shutdown/restart during this time, but that occurred in log.txt.2.