Header Flow Properties

[Flanderz]

Call me dumb but the idead of headers is to have the quickest path of flow from the engine to the back of the car? Right If so then why in alot of header design do I mostly see the last pipe crossing over the back of the other pipes to the other side??? Wouldn't it flow better if all pipes went straight down and out under the car?? or is it because of clearance issues? Just want to know ur thoughts?
EG:
http://i19.photobucket.com/albums/b164/skremnLS/Back1.jpg

[aimre]

its so when the pipes meets, the exahust puleses meet between one another. (if that makes sense)

[Boost]

thats what they call tune length.. for maximum power gains... exhuast headers work at its best at certain lengths... thus the reason for wraping the 4th pipe to the otherside.. those are hurricane headers.. not sure how good thoese will flow.. espcially at the merging points... for example looking at the collector from the shot you got up there... the transistion from 2-1 is not smooth