Re-installing Windows 8.1 on XPS 13 (late 2013 model)

Just went to re-install Windows 8.1 on a clients laptop and the only OEM discs I had from Dell were Windows 8.0. Not wanting to take longer to then install the 8.1 update I installed it from my Technet 8.1 ISO and I had a key ready but during the install it never asked for one and after I went into System in the control panel and it was activated already using the old product key. Where or how did the system know that product key? Not complaining just curious. Maybe it's in the hardware somewhere?