There are several possible reasons, but the most likely is what is known as Oil Formation Volume Factor (FVF for short, mathematical symbol capital B subscript little O). Basically, oil takes up more space in the reservoir than it does on the surface. The main reason for this is that oil in the reservoir contains large amounts of dissolved gas - possibly 1000 or 2000 cubic feet of gas per barrel of oil (say up to 300 cubic metres of gas per cubic metre of oil).

The gas molecules are small and fit in between the oil molecules, but oil with gas in solution is less dense and the oil simply takes up more space. Add to that the fact that the oil is up to 100 degrees Kelvin hotter at depth, so bigger, and then take off a little volume to allow for the high pressure downstairs, and you end up with Bo = 1.4; so for every barrel of oil you produce at surface you need to inject 1.4 barrels to replace what you are taking out.

1.4 is a very typical oil FVF - it can vary from 1.15 or so up to maybe 1.6 or 1.7, depending on a number of factors, principally the quantity of dissolved gas.

It is good reservoir management practice to inject one barrel of something for every barrel you produce. Here Aramco are producing oil, so they are balancing reservoir-conditions production by injecting 40% more water than they are producing oil at surface. I don't know what they are doing with the gas - they may be injecting it as well, in which case they are either over-injecting (and increasing reservoir pressure), or maybe compensating for an FVF of more than the 1.4 I mentioned.

When water starts to come through, the rate of water injection may be required to balance oil PLUS water production, so injection may have to increase further. Some of that extra 40% might be replacing that smll amount of water that they're getting already.

I don't know much about the history of Ghawar, so I don't know whether Haradh is far enough below initial pressure to need repressuring. So maybe they're overinjecting deliberately. Or maybe they've sized the WI plant extra big just in case of early water production, and they're taking advantage of it to overinject. With something that size you'd have to overinject like crazy before you did any harm.

4% is an acceptable inital watercut for an infill well, which this isn't, really. Probably means the water isn't quite where they thought it was. IIRC Ghawar is mainly reservoired in the Arab D Limestone, so a bit of water might easily cause scaling. It will be interesting to see if those expensive branched wells really make any difference to oil recovery in the long run.