OT: windows run as admin what's the difference?

[Tiemo Hollmann TB]

I know, that this should be addressed to a windows forum, but I know also,
that here is so profund knowledge in this list.

Since Vista there is the "run as admin" option to run a program. My only
knowledge about this is that the "rights are supposed to be higher" as only
to be logged on as admin. I didn't find yet anybody who could clearly
explain to me, WHAT excactly is the difference and when or for what you need
it. For example my customers have to install the windows quicktime player as
a requirement for my program. In 95% of cases, they just start the installer
and everything is fine, but from time to time I get a clean installation of
the quicktime player only with "run as admin". All cases are standard
personal vista or win7 computers with only one (admin-) user configured, so
no lack of rights from users side and antivirus guards switched off. I
understand that I need the "run as" option, when I am logged on as non
admin, to get the admin rights. But obviously there still is a difference
between "logged on as admin" and "logged on as admin + run as admin".  And
since years I don't understand what different things are going on when
installing quicktime as "log on + run as admin" and why is it installed
correctly in all other cases without "run as" Why does it work one time
without and once only with "run as admin" with the same installer?

Can anybody shed some light on this system topic? It is so frustrating to
pick in the dark.

Thanks

Tiemo