Different DCs Boot Differently?

I am now completely confused as I bought a new DC late January 2001 and a used one from Blockbuster recently and they're somehow different. The new DC works with every game I have whereas the used one works with most.

I've only found Plasma Sword and Lodoss to be 2 games that do not work on the used machine but fine on the new. Lodoss loads a black rolling screen and Plasma Sword load to the Sega logo, stay a while and then goes to the "Play, Settings, etc..." screen.

Please enlighten this poor and confused soul!