Space Invaders: why is the last guy odd?

Archived from groups: rec.games.video.arcade.collecting (More info?)

Hey all, this is sort of a tech question but mostly out of curiosity
not like I need to repair a machine or anything.
Is there a reason why the last guy left in Space Invaders moves faster
when travelling from left to right than when he comes right to left? I
have noticed it on every machine I have played. I assume there's a
timing issue or something?
Just curious.