That ensures symmetry but not equality. The issue is that the head loss change in the headers between pipes is different on the supply header vs the return header because the flow rates are different depending on the position of the node. On the supply side header, the flow rate starts off high and ends up low while the return header is opposite. So the pressure difference between branch pipes goes from high to low to high again meaning the flow rate through the pipes goes from high to low to high. I performed a nodal analysis for an 8 pipe example to show this effect and the chart below shows the flow rate difference between the pipes. The blue is for a 1/2" header and the red is for a 2" header. The total flow rate is the same for both setups but you can see how the 2" header is much more even than the 1/2" header. If you reduce the branch tube size, it becomes even more even.When it is getting primed, water will flow left to right in the feed line until the last panel. Once there is no more space, water will start to rise in the rightmost panel but soon enough pressure will also rise there. So water will start to rise in the before last panel and so on. Like a diagonal going right back to the left. Now on top, there will be more pressure on the right since all flow must go right. Which means that pressure on the top left will be less allowing water to flow out from the top left.