I discovered the same. I've been using log4j for years and it behaves
differently than logback in this context, it will roll the log
according to the configuration regardless of the execution context,
which is imho a better behavior. I hope it would be possible to fix
this issue or at least give an option between the two behaviors.

Here is how I use it:

• I have a monitoring task that is run every ten minutes and lasts
  for a dozen of seconds. I set logback to roll the logs everyday but
  it never happens because logback rolls ONLY if it is running while
  the date changes. I end up with a very huge log file that never rolls.
  Log4j will roll it during the first new execution after the date has
  changed.

More info: http://www.qos.ch/pipermail/logback-user/2008-October/000626.html