This is not extrange, the reboot command have options to avoid the generation of logs of the reboot event. This options are
-n or -d , from the man
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The reboot utility normally logs the reboot to the system log daemon,
syslogd(1M), and places a shutdown record in the login accounting file /var/adm/wtmpx. These actions are inhibited if the -n or -q options are present.
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One difference between uadmin and reboot is that with the uadmin command you can poweroff the system, for example "uadmin 2 6", with reboot you can't make this.
Additionaly , with reboot you can send options for the boot process, for example "reboot -- -r" to reconfigure, or "reboot -- second_disk" to indicate to the system that second_disk is the boot device, with uadmin I'm not sure that it could be posible.
I hope that it be useful.