Well /log/all never shows all log records; by default it always shows everything logged at INFO and above (after a fix I made a few months ago to not restrict it to hudson.* and jenkins.* namespaces); but then it was also showing FINE and lower records that were only being logged at all because other /log/* loggers had been configured to show them. This is because in java.util.logging, to receive log records you need to both set the level of the logger low enough and attach a handler to it, but these are separate acts: all handlers attached to the logger will receive the same records.
So this fix tries to avoid sending the finer log records to all when they are already being displayed somewhere else, currently just by limiting all to INFO and above. It probably needs some adjustment since you can also—IMHO quite confusingly—adjust levels of arbitrary loggers in the Jenkins UI without adding a /log/* logger.