Stop in for a cup of coffee

Good evening good folks.

I have something I wonder about, it's about light bulbs. Again, but this time it is different. I have discovered something,a I assume this is common knowledge to all Americans, but not to a Norwegian.
The 7 inch round headlamp for example, it seems to have several numbers. Like 60** whatever. What is the difference here ? Did they change the bulb number just for the sake of it, or ?
I wish someone could shed some light over this.

Bill