This appears to be a fixed-point (rather than a floating-point) format.
If you treat the 64-bit values as signed integers and divide by 4398046511104.0, you will get the decimal values you show.
e.g. the following will print -9999
int64_t x = 0xFF63C40000000000LL;
double y = x / ...
One word, precision
By definition, a normal is a vector with a length of 1.0f which means that each component of the vector will always fall between -1.0f and +1.0f, and UV are texture coordinates expressed in a range between 0 and +1.0f.
The number of possible states that you can represent with this range of numbers is huge and even exceeds the total ...