Your number 101.1 in base 10 is 1100101.0(0011) in base 2. The 0011 part is repeating. Thus, no matter how many digits you'll have, the number cannot be represented exactly in the computer.
Looking at the IEE754 standard for floating points, you can find out why the double version seemed to show it entirely.
PS: Derivation of 101.1 in base 10 is 1100101.0(0011) in base 2:
101 = 64 + 32 + 4 + 1
101 -> 1100101
.1 * 2 = .2 -> 0
.2 * 2 = .4 -> 0
.4 * 2 = .8 -> 0
.8 * 2 = 1.6 -> 1
.6 * 2 = 1.2 -> 1
.2 * 2 = .4 -> 0
.4 * 2 = .8 -> 0
.8 * 2 = 1.6 -> 1
.6 * 2 = 1.2 -> 1
.2 * 2 = .4 -> 0
.4 * 2 = .8 -> 0
.8 * 2 = 1.6 -> 1
.6 * 2 = 1.2 -> 1
.2 * 2 = .4 -> 0
.4 * 2 = .8 -> 0
.8 * 2 = 1.6 -> 1
.6 * 2 = 1.2 -> 1
.2 * 2 = .4 -> 0
.4 * 2 = .8 -> 0
.8 * 2 = 1.6 -> 1
.6 * 2 = 1.2 -> 1
.2 * 2....
PPS: It's the same if you'd wanted to store exactly the result of 1/3 in base 10.
