Simplest explanation: The owners had them installed to test different contractors, designs, and/or materials. Wouldn't you have someone put a few in before you signed a contract to put them all up?
Also, maybe they're testing window designs and not checking out the contractors work. "Does it fit?" "Let's order 3 and try." It's not hard to imagine why someone would want to install a few windows in a big abandoned building before installing all of them.
I've seen builders make little brick monoliths in a vacant lot to test the look of different materials.
I think these were likely installed at the expense of a window materials salesman.