msiexec /a != Run as Administrator
/a
tell the windows installer to to make an administrative install point (AIP), which is like creating a new, customized install image. You, generally, can preconfigure some installation choices for the AIP. Then, when someone installs the software from the AIP, all those choices are already made. Usually, you'd make the AIP available over the network, and client computers would install the application from there. I think you can also apply updates to the AIP, and all the clients that use the AIP will install the updates automatically.
This has nothing to do with "run as Administrator," which is about process elevation. msiexec.exe will usually need to run in an elevated context, but you either need to do that outside the script where you're invoking msiexec or have the script start an elevated process. (Some ideas for that can be found at: How can I auto-elevate my batch file, so that it requests from UAC administrator rights if required?.)