That item is user-changeable (Yes/No)
Some applications are not compatible with multi-core, which is why that setting exists. However, on a Windows PC it's more usual to configure a multi-core processor (when required) by using the System Configuration Utility (msconfig) as it gives you more precise control rather than simply enabled or disabled.
I imagine the BIOS default is "enabled" (just a guess) so maybe a previous user has changed it?