Pelican Pete

My wife's Subaru Impreza uses H4 headlamp bulbs, and I needed to replace one of them.

The manual clearly states that they are H4 bulbs.

So she stopped by the auto parts store on her way home from work (across the street from her job) to get one. The guy at the store sold her a 9003 bulb.

My '84 Saab also took H4 bulbs, but I haven't driven that car since 1996, hence I haven't used H4 bulbs since then either, and I didn't remember what they looked like.

Turns out that the 9003 was indeed the same bulb as the H4 which I took out of the car. I just want to know WHY would they change the name of the damn thing? Or is that just Sylvania's nomenclature?

Thanks,
Peter