Hi , The reason is 6 is written in binary of 32(000.....110) bits , then all the bits are complemented(111........001) which makes it negative number. Using 2s complement notation(flip all bits and add 1) you will get -7. Hope u understand.
Or if you prefer, you can memorize an equation- do you see the relationship between the above numbers and the positive numbers?
To go from negative to positive, you invert the bits and add one To go from positive to negative, you invert the bits and add one (1 inverted is 11111111111111111111111111111110, and you need to add one to get to 11111111111111111111111111111111)
So now you are inverting the bits on the number six. You have not added one. If you did add one, you would have -6. So -6 missing 1 is -7.