Why building a house almost always costs more than calculated

That was a special fabric made of polypropylene tapes.

Just from the image comparison (Google) it looks similar. OK - there are differences. I think the material with the ribbons are probably the dark reels (at least you can see a ribbon structure here if you look closely). I sent you the offer from a large internet retailer starting with "A" via private message.

Due to extremely heavy rainfall, additional work became necessary for us. Here is an example of additional earthworks on the basement beyond the standard service:

Disposal of 600m³ of excavation: €8000
150m³ of backfill material: €1050
Pump shaft for open water retention including pump rental: €1500
Light well emergency drainage: €1500
Construction site access and crane area: €1200
Soil survey with foundation recommendation due to questionable load-bearing capacity after the rain: €1200
Geotextile and 15 cm additional gravel layer under the floor slab: approx. €1800

Isn't this about additional services beyond the standard services?! Why is then less material reinstalled than excavated and disposed of? I assume that this is softened clay, loam, whatever caused by the rain and not the standard excavation for the basement? How long was the pump on the construction site for €1500? Were you allowed to keep it? Construction site access and crane area should rather be "standard services" or if the crane area was specified as construction provided in the construction service description, then it should have been included in the calculation. Why was the gravel layer under the floor slab additional?

Sorry, I just want to understand, since this is about extra costs...

This is about additional services compared to the standard services, right?! So why is less material reinstalled than excavated and disposed of? I assume this is about clay, loam, whatever softened by the rain and not the standard excavation for the basement? How long was the pump on the construction site for 1500€? Were you allowed to keep it? Construction access and crane area should also rather be "standard services" or if the crane area was listed as provided by the builder in the construction service description, then it should have been included in the calculation. Why was the gravel layer under the foundation slab additional? Sorry, I just want to understand since this is about additional costs...

The groundwater had to be lowered to 1m below the foundation slab level. For this, a deep hole was dug and 4 concrete rings were installed and filled around with gravel. The rings had to be installed with the crane. The rental for the pump was 30€/day.

We were aware of the costs beforehand; it was also mentioned several times by the construction company, and we received a list of required items in advance, so we were able to estimate the costs well in advance.

And yes, these are ADDITIONAL COSTS that are hardly included in any construction service description. There are many who, although being informed by the construction company, simply think "our soil is good, we don’t need any fill material, etc."

Oh yes, crane space etc. I already mentioned during the initial conversation. He said it simply varies too much depending on the situation. One needs a road closure, the next 50m access. If I remember correctly, he quoted me €25/m² for excavating 30cm deep and installing and compacting material. But we needed more material because the ground was like jelly after the heavy rains. If we started today, we wouldn't need any of that. Everything is dry and hard.

We were aware of the costs beforehand

Well, that's good...