Why is SCHAR_MIN defined as -127 in C99?

§5.2.4.2.1 of C99 defines SCHAR_MIN as -127 and SCHAR_MAX as 127. Should not the range for an 8 bit signed integer be -128 to +127?

The limits.h for my compiler defines SCHAR_MIN as (-1 << ((CHAR_BIT)-1)), which is -128 given CHAR_BIT is 8.

Is there any reason why SCHAR_MIN was defined -127 and not -128 ?