Why does 1 represent a negative sign bit?

[Cinder]

It seems like 1 (a positive number) would represent a positive sign and 0 (a non-positive, so positive or negative a negative number) would represent a negative sign. This is especially apparent when converting unipolar values to bipolar values (e.g. 3, 4, 2, 1 instead of 1, 2, 3, 4), like when converting string to integers and vice versa (I.e. you would just put a leading zero in front of the number to negate it, instead of so much as having to use a fixed number of bits to represent signed integers).

Any insight for me?