Question about dash lights
 

I have a 1989 206 and it has the VDO instruments. Some of the bulbs were out so I pulled an old bulb out an it had on it 3w 24v I assume that to be 3 watts 24 volts. So I ordered up the same bulb. When I installed them they were about half as bright and the other bulbs. So my question is will a 24v bulb be half the output of a 12v bulb when connected to 12v? The reason I ask is I can get the same exact bulb 3w 12v vs. 3w 24v. I just don't know why the other 24v are brighter.

Thank for any and all help.