Behaviour of Matlab fixed-point numbers with 1-bit

I noticed a funny behaviour with Matlab-FI numbers. It occurs only with 1-bit, signed numbers (a datatype that does not make a lot of sense in practice). I'm aware that multiplying a double with a fi is not a good practice and the behaviour may even depend on the Matlab version, but I would expect the behaviour to be same independent on the number of bits. This seems to be consistent in versions 2011b, 2013a and 2013b and I would like to understand why it happens that way.

*Example 1:* -1 can be representet as 1-bit, signed. However the result has a fraction-part of -2 which would allow only numbers [-8, -4, 0, 4]

    >> a = fi(-1,1,1,0)
    a = -1
          s1,0

    >> 1*a
    ans = 0
          s2,-2

*Example 2:* A 1-bit, unsigned behaves as expected, the fraction-part is unchanged

    >> b = fi(1,0,1,0)
    b = 1
          u1,0

    >> 1*b
    ans = 1
          u2,0

*Example 3:* A 2-bit, signed behaves as expected, the fraction-part is unchanged, the integer-part is extended

    >> c = fi(-2,1,2,0)
    c = -2
          s2,0

    >> 1*c
    ans = -2
          s4,0

*Example 4:* Another 1-bit, signed showing a similar behaviour as in example 1.

    >> d = fi(-0.5,1,1,1)
    d = -0.5000
          s1,1

    >> 1*d
    ans = 0
          s2,-1