How does my 2007 MacBook have El Capitan running on it?

Hi, I recently purchased 2 2007 MacBooks used.

One each for my wife and I. Mine has OS X El Capitan running strong on it and I have no problems with speed, WiFi or graphics like some forums suggest . 4gb ram 120gb hdd Hers on the other hand same build (4gb ram 120gb hdd) I try to upgrade to El Capitan via bootable usb and all I get is the prohibited.

Mine has an upgraded ram and hdd

Hers only has an upgraded ram could it be the original hdd? If I replace it could this be the problem?