I realized the difference between value and representation of value.
Or to put it the other way round, a computer manipulates bit patterns, it is the interpretation of those bit patterns that give them meaning.
For example a bit pattern can have a numeric value if you interpenetrate it one way and that bit number can be expressed in any number of different number basis. However the same bit pattern can be interpreted in a different way to mean a number that ranges over positive and negative values. The same bit pattern might be interpreted as a floating point numeric value or a fixed point numeric value or a BCD (binary encoded decimal) value or even an alphabetic character according to any one of a number of conventions like ASCII, Murry Code, Mores Code ... the list goes on.
Just remember it is only bit patterns not numbers that a computer manipulates.