pam_unix(cron:session): session opened for user root by (uid=0)
Solution 1:
Ubuntu uses vixie-cron, which, in addition to /var/spool/cron, also looks in /etc/crontab and /etc/cron.d/*.
<Edit> According to your update, you have four cronjobs in /etc/crontab. They are being run by cron on their specified times, and that's what causes the syslog messages.
cron itself does not know anything about /etc/cron.hourly/, it just runs run-parts, which handles said directory. As far as cron is concerned, the cd / && run-parts --report /etc/cron.hourly job is exactly like another job, scheduled to run hourly. It is not magically skipped.