The mw-maintenance cscript 'parsercachepurging' (class mediawiki::maintenance::parsercachepurging) is scheduled to run once per day.
The full command line is:
0 1 * * * /usr/local/bin/mwscript purgeParserCache.php --wiki=aawiki --age=2592000 --msleep 500 >/dev/null 2>&1
Note how it has the msleep parameter and runs once per day.
When running it manually i noticed it is very slow. So slow that after a couple hours running it in screen i was still only at like 6%.
This says to me it's almost certainly not getting to 100% before the next run is already scheduled. Possibly then running twice.