I've noticed this too, and have been watching my phone to try to understand what's going on.
The icons on the right are, according to the HD manual, "Status Icons" and I've seen that the sync icon appears here, for instance, when the phone is syncing with online accounts, as defined in Settings > Accounts & Sync. I think some other processes also bring up this icon and, although I'm not sure of this, these seem to be the things that are closely tied into the Android/HTC operating system.
The icons on the left are "Notification Icons". This is the area where applications put their icons. So, if I'm right, this is where any applications you've installed (or that aren't integral to Android) that do a periodic sync will place the sync icon.
Some of this is conjecture, but it seems to make sense.