Magic Brush / Zauberpinsel: even lowest tolerance is marking nearly the whole image 14.4.1.6

Hello,

a) when i use the magic brush, often there is to much of an aerea selected - even if i choose "tolerance = 0 or 1 or 2" 

Is there anything to do, to change that behaviour?

b) Often it is not possible to unselect parts of the magic-brush-selected. I use the eraser tool, but after deleting, the  stupid "magic" is taking over again.  Greetings from the Terminator.... Machines are ruling.

Can there anything be changed?

thanks for help.