OK, this one is definitely NOT a result of my own incompetence.
Yesterday after setting the language to english (thanks to Xaver for the hint) I started a new game and my initial soldiers had outstanding to superhuman scores in each n every weapon related stats (close combat, heavy, asssault etc.). I quit and started a new game three times and had the same effect.
THEN I quit the application, started it anew and my next pack of soldiers had normal ability scores. HOWEVER after the first month the additionally available soldiers once again came with those superhuman scores.
Looks like there is some serious problem with the random numbers generation process for the starting ability scores.
edit:
After reviewing the bug tracker on sourceforge.net I have to add that I did never select an easy campaign before or afterwards like others who encountered that phenomenon have.
I ONLY tried the standard campaign.
So my guess is that for some pecular reason new soldiers sometimes come with initial stats that should only appear in the easy campaigns.