I've played whole games never bothering to actually do the calculations and just working with the amounts without a hitch.
However, I am now going to take you by the hand along to a journey into the expected values of using Traffic Signs, and you'll see the issue!
So, 400 points, and this tooltip can be read 2 ways:
A) Stop 20% of (the LP leaving the Hex in this direction)
The LP leaving the Hex in this direction = 200.
20% of 200 is 40.
Stop 40 LP of the 200 LP going this direction.
So I expect 160 LP to continue and 240 LP now going the 1 other direction.
B) Stop 20% of (the LP) from leaving the Hex in this direction
The LP = 400.
20% of 400 is 80.
Stop 80 LP of the 400 LP from going in this direction.
So I expect 120 LP to continue and 280 LP now going the 1 other direction.
Now a reality check:
177? That wasn't an option!
It's closest to interpretation A though. Which to me also makes the most sense anyway from the understanding I have of the system.
And it's suspiciously close to 160 + 16 + 1.6 ( = 177.6).
So is this an indication of a recursive function calculating these?
Let's try another number:
We'd now expect 80% of 200 = 160 to be blocked from the 200 going this direction (interpretation A).
So 40 goes through. And then somehow 40 + 4 + 0.4 = 44.4 to actually be the amount that gets through.
... that's not good!
Interestingly, it's now closer to interpretation B where 80% of 400 is 320 and so blocking 320 of the 200 (impossible) means blocking it all, leaving 120 leftover that gets added to the free branch making that branch, NW, take 120 more points.
120 + 12 + 1.2 plus the original 200 is indeed the 333.2 points that are leaving in that direction.
Then, to avoid obvious bugs, send the leftover down to the SW (66 points (rounding)).