That is because they are represented differently in the computer's memory. This notion of representation is very important. In the Western World we usually represent the value 5 with the symbol '5'. But, we also know that the Romans used the symbol 'V' to represent the value 5 while in the Arab world a heart represents the same value. In other words, there is a difference between a value and the symbol or symbols used to represent that value.
Computers have their own representational systems for integers, doubles, and characters.
Integers and doubles are usually represented with the binary number system . Computer memory is broken up into bytes with each byte having 8 bits. The bit is the smallest memory element, consisting of one 'switch' that can be 'on' of 'off''. More precisely, these on's and off's are represented by 'high' or 'low' voltages in the hardware for a bit. We tend to think of these on's and off's as 0's and 1's.