I'm guessing it's things waking up periodically.
When I worked for Orange on smartphones htc supplied an application which records when it wakes up to each wake-up level, so we could easily see if our customization apps were stopping the phone sleep properly.
I was thinking this morning that we may need something similar.
The different "levels" of suspend that Windows Mobile and presumably Symbian use don't really apply under Linux. If the processor is configured to wake on a given interrupt, it wakes fully. You can find out how often this is happening by looking at syslog:
grep htcuniversal_resume /var/log/syslog
You should find it only wakes on a GSM event (call or SMS), an alarm, or when you press the power button.
I know there are problems with the wifi driver's power management - not least that the kernel powers up wifi by default (loading the htcuniversal-acx module powers it down again). There may be more.
Edit: It looks like the bluetooth module is powered up by default. I've updated titchy-hardware-support to turn it off. I've no idea whether it will make much difference, though.