I think it looks odd, too. If you go with this:

    ADC = A + V + C
    SBC = A - V - (1 - C)
    SBC = A + (-1 * (V - (C - 1)))
    SBC = A + (-V + (C - 1))
    SBC = A + -(V + 1) + C

"The SBC instruction subtracts a value from the accumulator register, with an extra 1 subtracted from the result if the carry flag is NOT set."

The explanation fits though. If the carry flag is not set (C == 0) then you subtract an extra 1. If C == 1 then it's A-V.