Hi,
It looks like that's because the number is negative, while the previous number was positive.
It looks like another way to compute this when the number is negative is as follows:
11000
where the leftmost bit is the sign bit and this number is negative, you take the right 4 bits:
1000
and that equals 8 decimal, so the number is -8. Pretty simple that way.
This means zero can be represented as 00000 or 11111.
Positive 8 would be 01000.
I've never actually had to use 1's complement in any programming i ever did since the mid 1970's when microprocessors where just getting popular, even in specially designed programming equipment where we had the choice of what system to use. I've always used 2's complement for everything, including assembler, Pascal, Basic, C, C++, Windows API, etc.