command line - using "runas" with "Administrator" account versus other admin accounts

On my computer, I have two admin accounts. The default admin account named "Administrator", and a new admin account named "testadmin."

If I run the following commands:

runas /user:testadmin cmd.exe
runas /user:Administrator cmd.exe

The first one opens up a new cmd.exe as "testadmin", but it's not truly running in Administrator mode, i.e. I get "Access denied" if I try to write to C:\Windows.

The second window opens up a new cmd.exe as "Administrator", but it's running in true Administrator mode. I can successfully write to any folder on my computer.

The name of the first window is "cmd.exe (running as [ComputerName]\testadmin)"

The name of the second window is "Administrator: cmd.exe (running as [ComputerName]\Administrator)"

If both accounts have admin privileges, why is there a difference in behavior here? I hate that I can't open a window from the command line as testadmin in true Administrator mode... it's really annoying.