I don't know if this depends on Loaders Modernized mod or on Pyanodon's mods.

In a complex base I have a Wood processing unit MK1 with TURD enhancement that produces 40 woods per second. Since I only have yellow belts at the moment, I want to take as much wood as I can from the unit, so I am using two unloaders. However, if I put the unloader on the direct output of the unit, I find that a unit of belt has 12 items instead of 8. It seems that it add the 4 woods on the direct output to the 8 woods of the unloader.

How to reproduce:
1. Select the TURD "selective cutting heads" from the fastwood forestry upgrade TURD.
2. Load a Wood processing unit with enough logs so that it never starves
3. Put a yellow unloader on the direct output, then follow it with a few belt pieces
4. Ensure the yellow belt always unloads at full speed
5. Connect with red wire a pole to a belt section, then select Hold as read content mode
6. See that the red wire reads 12 woods, while I expected that to be 8.

I do have a screenshot, but I don't know how to upload it.

OK, this is really interesting! Wouldn't have thought of it until I saw it, but I think this is what is happening.

You likely have Transport-belt-capacity-1, so you can stack 2 high on belts.
The Wood processing unit is able to stack it's output on belts, but as you know it only outputs to one lane.
In Loaders Modernized, your mod setting likely has stacking to None or Separate Stake Loader, so the yellow loader will not stack output.

By placing the loader directly at the Wood processing unit's output, the Wood processing unit is outputting stacks to the lane it outputs - directly onto the "belt" portion of the loader. The loader is pulling wood out of the Wood processing unit to place on the unfilled lane - but not stacked.

So, a stack of 2 high (8 total) on one lane and 1 high (4 total) on the other lane. That gives you the 12 items per belt segment. When you get to Transport-belt-capacity-2 you will have 16 per belt segment 12 [4 * 3 high] and 4 [4 * 1 high]).

Your reply is interesting, but it doesn't explain what happens if I put a splitter after such belt. See here: https://forums.factorio.com/viewtopic.php?p=705285#p705285 . And, btw, you can see there two screenshots, where the left belt is graphically equal to the right one, while I supposed item stacks would be visible (I've never user stacks on belts, yet).

If you're playing modern pY and have the mod "Enable All Feature Flags" you have belt stacking enabled. There are other mods that would enable that feature too: Stack inserters, space age, and a few others.

Your image does show a stack of 2 (you have to zoom in to see it) on the right lane of the left belt (before the splitter), which is exactly as I explain above. Splitter mechanics and how they handle full lanes is discussed elsewhere (https://forums.factorio.com/viewtopic.php?p=585390&hilit=splitter+mixing+lanes#p585390), so I won't go into depth here. However, the splitters are operating exactly as expected in the images you posted.