I have almost two identical tree containing polygon lines. The only difference is that there is one extra branch in the second one. I apply identical commands on both, but the branch structure at the output is different. Why?
Nice question. I couldnt find a problem in your gh file.
As i said i dont know the reason why this is happening but if you simplify the extra branch data you have the same format with the other.
The List Item below is prioritising the first input tree structure but the List item above is using the second input structure. You seem to have found the weak point of that heuristic which chooses which data structure to eject when the inputs have compatible but not equal structures.
Or perhaps it is proof that new physics exists beyond the standard model.
I confirm the different outputs for the List Item even after clean-reinstalling windows and rhino two times
Nobody wants to solve the mystery of the century?
You have 2 things to do. Live with that small problem and submit a bug to the McNeel team.
It is a Grasshopper core problem so it seems out of reach for USERS.
To live with that is not so hard. Right click on output “i” and select “Simplify”. I am sure you can do it.
I know simplfy. Unfortunately I am using the List Item index in several places in my script. So that is not a solution. But thanks anyway. Actually you just suggested a workaround. So the mystery is still not solved. Actually I see that sometimes McNeel developer’s answers here. It is just accident?
You can insert some message targeted to specific persons, but at this time David Rutten is surely working on others things (Grasshopper 2 Alpha stage)
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This is quite a subtle bug. The list item component has its own, special list matching behavior — the heuristic @Dani_Abalde mentioned for picking which structure to use — which slightly alters its list processing for lists larger than 50 items, presumably as part of some optimization for large lists.
To ensure that it always uses the same input to drive its choice, the best option is to right click the input you want to drive the structure and designate it as “Principal”: 
Then you will get consistent output regardless of the length of the input lists.
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