Where did he say that?
Anyway, there are two possibilities:
1. There is resistance to the flow by something like a radiator.2. The pipes above that end with a U, before the water goes down to pipe that returns to reservoir.
I meant Smith, not Murray... A resistance would reduce pressure but not mass flow rate. I suppose a U, or some weird bends could do that, but I haven't seen anything like that on a photo or diagram.
The flow meter would work if it were installed in a U, but it wasn't.
In a sloping pipe like shown in the photos, as long as the meter below the level of the water in the reservoir, then that's really the same thing - ie. a U isn't always fundamental to getting a good measurement.