On apt 0.8.16~exp5ubuntu13, theres this:
[ Adam Conrad ]
* On armel, call update-apt-xapian-index with '-u' to keep the CPU
and I/O usage low. We would do this on all arches, but there's a
regression risk here, but that's better than killing slow systems.
Shouldn't the daily script run with -u *always*, and let the re-built exclusively for the weekly script?