2000 GMC Sierra Daytime Running Lamps

I am in automotive school and I replaced my burned out Daytime Running lamps the other day on my 2000 GMC Sierra. My teacher noticed something about the bulbs. They are dual filament bulbs. However, they only run in daytime and at one brightness. Anyone have any idea where I can get info on this as to why they are dual filament? Its worth extra credit for my grade : )

I am not 100% sure, but I believe one is for daytime running lights and the other is for the headlights. The DRLs aren't as bright as the headlights.

I think the DRL bulbs in my 03 are dual filament as well. Mine don't light when the headlights are on though.

As a side note, the third brake light bulb on our Impala SS is also dual filament but only one side lights. Interesting issue.....