Yeah I can see the confusion there. Space Invaders by todays definition is a (single screen) Shooter so it's valid there.

Arcade was on the list before, but was quickly taken off as it can not be clearly defined now-a-days. It's too vague a category as you can now make anything into an 'arcade game' and charge people quarters for play. Then again there is the other aspect of it, it is a classical game like pac-man or Space Invaders? Or is it instead a fighting game like Street Fighter or Mortal Kombat?

See what I mean?

But yes, Space Invaders would likely fall under Shooter. We just don't have the single screen shooter up on the sub category list. I guess I could/should add it for clairity.