[2.0.21] inconsistent behavior of red splitter

[makpia]

i am aware of this issue from https://www.youtube.com/watch?v=3iVuNEElaxc and then did some tests myself.
this issue is highly related to but different from 119856. i am not pointing out the differences between 1.x and 2.0, but the differences between the red one and other colors

details of screenshots:
1 - 'directly output machines' like drills directly output to yellow, blue and green splitter will be faster than to belts. except the red one. the green light in the screenshot means items at left side is more than the right side.
2 - inserters output to yellow, blue and green splitter will be faster than to belts, except the red one.
all machines start at the same tick

this behavior is against the response of boskid in 119856. to my understanding, the fastest belts(the green one) moves (60 / 2) / 4 / 60 = 0.125 tile per tick, which is not enough to reach something called 'the head of the transport line'(i have no idea what it means) that is '0.2 tiles away' in 1 tick. so items will not reach 'the head' no matter what the colors is and thus all color should behave the same. and the green one can even output full lane with no gaps.
or maybe i misunderstood what boskid is saying, i will appreciate further explanation