i figured this out:
If i place 10 blocks of 0.1 they have together not the lenght of 1. They are together ~ 9.997 long. That is not much, but it makes a small gap.
If you place the 10 blocks with the magnet, you get a bigger gap: The gap is 0.04 big. So the 10 block are together 9.96 long.
So the magnet-gap is also visible, if you dont search for gaps.
I wrote this thread, because it helps to understand how gaps arise.
The picture shows the 9.96-gap (on the right side of small blocks).
Edit: Only with modular-buildung 10 small blocks are not so long like a big block. With non-modular-building you build exactly or so, like i explained. Thanks to Minotorious for the hints and explanations.