TWF vs THF

This is not a comparison, but rather a question. Everyone says that TWF is better, but I question the math that many people use, because I think there are some mis-conceptions (maybe mine).

I have read people say that 2 weapon hitters do more damage, because they have 2 weapons, so double the bonuses, double the procs, etc. However I don't think this is true.

If they are using 2 weapons: 1 weapon hits, then the other. If they are using 1 weapon, the same weapon just attacks twice. Same number of bonuses and same number of Procs.

I realize that 2 weapon fighters get 1 extra attack in the chain, and more and more as they add feats, so here is the second part of my question: Does that really matter? In PnP it obviously does, because combat happens in rounds, but in this game, with realtime animation, does swinging 6 times as part of one attack chain really amount to more damage than me doing 3 chains of 2 attacks each in the same amount of time?

I am not trying to start a thw is better thread, because on paper, it obviously is not in most cases, however I am saying that maybe the real-time aspect introduces factors that cannot be represented "on paper" (Mostly to see if twf builds are so much better that it is the only viable build for high-end dps - because it seems to me that a weapon and shield should be the same (again same weapon attacking twice as many times), and two handed would be at least as good, with the strength bonuses)

But I could be missing something...