Why can my app write successfully to HKLM\Software under Vista?

Everything I read consistently tells me that if UAC is enabled under Vista
(which it is on my PC) an ordinary application (post-install) which does not
have any kind of manifest at all, cannot write to the HKLM\Software area of
the registry. Instead, what is supposed to happen is that Virtualization is
supposed to redirect the write to

HKEY_CURRENT_USER\Software\Classes\VirtualStore\MA CHINE\Software

Well it doesn't. I run my app and change a per-machine setting and it
updates HKLM\Software no problems. Not only does it look to be updated fine
if I look at the HKLM\Software area in RegEdit, it still looks OK if I log
off and log back on using an ordinary (non-admin) account and look at
HKLM\Software there. If I run my app and change the per-machine setting
using the other account, sure enough it is changed when I log back on using
my first account.

In my first account, there do appear to be some settings in the
'VirtualStore' area but not many. Most of the settings appear not to be
virtualised.

What is going on? Any help with this would be much appreciated. The
documentation I am reading just does not match what I see.

Simon