Sometimes it’s the cloud thickness (min / max ceiling) and the layers being close to each others create this dark effect.
Light won’t go through these.
If you create manual overcast and play with the parameters I mentioned you will see how the land turns dark when layers are thick and close.
In real weather scenarios if such cloud settings are met, certainly it will be dark.
Overcast with a diffused bright light is usually the result of a thin overcast cloud layer and no other layer close to it on top.