Permaculture, Sustainable Design, Homesteading, Off-Grid Living, Natural Building, and more

I am building a laundry to landscape greywater system. I have seen some people in some videos that are using T fittings as 50% splitter like in this image:

(taken from here)

You can see that the water that flows from the source pipe is being split to two destinations:

1. The basin that is watering each tree.
2. The continuation to the next tree.

My question is, is T fitting actually considered a 50% splitter ? Because I would imagine water will want to continue flowing in a straight direction, so most water will continue flowing and not "turn" 90 degrees in the pipe for the tree basin.

Remember that water is not going in pressure, but with gravity.

So why do I see so many of these designs and are they correct ?

Thanks