Precision M6500 Has Desktop Service Tag In BIOS

I recently bought a used Dell Precision M6500.  It looks good and seems to be operating fine but when I hit F2 and went into Setup, to change the time settings, it showed a different Service Tag than what's on the bottom of the laptop.  In fact, the BIOS Service Tag says that the laptop is a Vostro 220 desktop!! What's up with that?