The cross-sectional area answers this.
(i.e. can't fit two adjacent pipes plus the gravel into this same cross-sectional [CS] volume).
The Polyethylene plastic sheet left a perfect forensic outline of the CS volume. What I suspect happened is that when the heavy concrete pour was vibrated, the plastic "tent" design deformed where the "marble like" round rock actually went under the drain pipe to the other side, forming the adjacent hump. That's how the plastic ended up wrapped around the one side of the drain pipe & no drain rock on that side - and most/all of the rock on the other side (an voila, the "hump").
This design is very unstable to overpour concrete & vibrate it - as you can see from the results.. I'm surprised someone didn't think of what would happen.
I was thinking more in terms of they laid down 2 pipes for whatever reason in that spot. Then poured. The concrete will, depending on viscosity, flow to fill around it regardless of the drawing. In other words, I was wondering if they deviated from the drawing to accommodate two pipes for some reason.
Keeping in mind that I’m a bit of a novice in this area. I can certainly see how the vibration/shaking process could dislodge things and leave voids where they once were.