What is the 2's complement answer of 16.5?
It seems that WolframAlpha is using fixed-point representation. In effect, it is doubling your number and then representing it as two's-complement.

In more detail, WolframAlpha is guessing that you are interested in fixed-point representations of the form
$$
xxxxxxx.x
$$
in binary, that is, 8 bits in total, with the "decimal" point at the fixed position between the 7th and 8th bits. Using two's complement, you can represent all numbers from $-64$ (represented as $1000000.0$) to $+63.5$ (represented as $0111111.1$).