Because the fluid had to flow.

Before Factorio 2.0, fluid moved through a pipe like real water, piece to piece. Wube's Friday Facts (raiguard, June 2024) puts it plainly: pipe throughput "decreases over distance at an inconsistent rate", it "can vary depending on the order that the pipes were built", and at a junction one side could get more than the other. "The issue was not necessarily that this behavior existed, but that it was unpredictable."

Fluids 2.0 stops making the fluid travel. Pipes, underground pipes and storage tanks merge into fluid "segments", and "segments are a single unit": fluid pushed in "will be immediately available at any point along a segment", and a machine pulls at a rate proportional to how full the segment is ("if a segment is half full, then the pulling rate is half of the maximum"). Junctions stop mattering, and splits come out much more even. raiguard calls it "a fairly large step back in terms of the realism", but pipes "just work".

The factory, the pipes and the numbers in this film are our own simplified version, built in code from the post; the post gives no final numbers, so the sizes, rates and percentages are ours.

Which game's hidden rules should we cut open next?

Sources: raiguard, "Friday Facts - Fluids 2.0", Wube Software, 21 June 2024 (factorio.com/blog/post/fff-416).
Sounds: our own, made in code.