Hi all, can I please get some help with di2 setup for an IA?

I have read some of the other posts here about it, but am a bit confused as to why the suggestions seem different from my current Trek SC9...

So with my trek, I have both the di2 brake levers, and the di2 aero extension shifters.
The 4 wires from these all go to one 5-port junction box that I have close to where the cables go into the top tube.
From there, one wire goes down to the BB where there is a 'B' 4-port junction.
From the 'B' junction, one wire goes up the seat tube to the battery, one goes to rd, one goes to fd.

So what is different about the IA setup, other than the fact that I believe I can put the battery in the batpac (or whatever its called) behind the bb??
If it matters, the two bikes are basically same size - medium trek to a 54 IA.

Thanks!!!

EDIT - I just looked at the supp manual online (duh...).
https://www.feltbicycles.com/...talManual_102315.pdf

So I see on the first page, the only diff from my above setup is the extra junction b box in the head tube.
But I'm not clear as to why. Are the bars on the IA longer than on a typical trek, and thus the wires from the brakes/shifters aren't long enough??

And - is it different for the IA1 & FRD style frames? When I look at the specific guide for those, they really just show the wires going into a 5 port junction in the head tube, but I don't see mention/pics of an extra 4 port junction in there..


Thanks!