China DOS Union

Please look at this
@echo off
echo set123456789>123.txt
set b=123456789
echo %b:~0,5% >>123.txt
echo %b:~-5% >>123.txt
echo %b:~2,5% >>123.txt
echo %b:~-5,3% >>123.txt
echo %b:~-5,-4% >>123.txt
echo %b:~-5,-3% >>123.txt
Why is it that
echo %b:~0,5% >>123.txt
echo %b:~-5% >>123.txt
echo %b:~2,5% >>123.txt
echo %b:~-5,3% >>123.txt
directly display what they extract,
but
echo %b:~-5,-4% >>123.txt
echo %b:~-5,-3% >>123.txt
display what remains after their extraction?

set123456789
12345
56789
34567
567
5
56

The result is like this? That's perfectly normal. I can't see what the problem is.

About "string extraction": %a:~]%
　　The square brackets indicate optional parts, % is the variable identifier, a is the variable name and cannot be omitted, the colon is used to separate the variable name and the description part, and the symbol ～ can simply be understood as "offset". m is the offset (default is 0, meaning extraction starts from the 1st character), and n is the extraction length (default is all).

%a:~0,n% takes the leftmost n characters
%a:~-m% takes the rightmost m characters
%a:~m,n% takes n characters starting from position m+1
%a:~m,-n% starts from position m+1, up to the (n+1)th character from the end
%a:~m % takes everything to the right starting from position m+1.