More interesting is exactly why floating point 0.1 + 0.2 != 0.3, and it's due to the way rounding is defined in the IEEE-754 standard:

> An implementation of this standard shall provide round to nearest as the default rounding mode. In this mode the representable value nearest to the infinitely precise result shall be delivered; if the two nearest representable values are equally near, the one with its least significant bit zero shall be delivered.

If we convert from decimal to double precision (64-bit) floating point, here is how they are represented in hexadecimal and binary:

    0.1 -> 0x3FB999999999999A = 0011 1111 1011 1001 1001 1001 1001 1001 1001 1001 1001 1001 1001 1001 1001 1010
    0.2 -> 0x3FC999999999999A = 0011 1111 1100 1001 1001 1001 1001 1001 1001 1001 1001 1001 1001 1001 1001 1010
    0.3 -> 0x3FD3333333333333 = 0011 1111 1101 0011 0011 0011 0011 0011 0011 0011 0011 0011 0011 0011 0011 0011
                ^^^^^^^^^^^^^                  ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^ ^^^^

Taking 0.1 as an example, here is what its binary representation actually means:

    sign exponent    mantissa (marked using ^ in the table above)
       0 01111111011 1001100110011001100110011001100110011001100110011010

The exponent is encoded as its offset from -1023, so in this case we have 01111111011 which is decimal 1019, making the exponent 1019-1023 = -4.

The mantissa (BBBB…) is an encoding of the binary number 1.BBBB…, so with an exponent of -4 that makes the actual number 0.0001BBBB….

Applying this for each of these numbers:

    decimal  binary
    0.1      0.00011001100110011001100110011001100110011001100110011010
    0.2      0.0011001100110011001100110011001100110011001100110011010
    0.3      0.010011001100110011001100110011001100110011001100110011

Then if we add 0.1 + 0.2, this is the result:

      0.00011001100110011001100110011001100110011001100110011010
    + 0.0011001100110011001100110011001100110011001100110011010
    -------------------------------------------------------------
      0.01001100110011001100110011001100110011001100110011001110
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

However we only have 52 bits to represent the mantissa (again marked with ^), so the result above has to be rounded. Both possibilities for rounding are equidistant from the result:

      0.010011001100110011001100110011001100110011001100110011
      0.010011001100110011001100110011001100110011001100110100 <==
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

So according to the specification, the option with the least significant bit of zero is chosen.

Converting this back to floating point format we get 0x3FD3333333333334. Note that the least significant four bits of the mantissa are 0100, which corresponds to the trailing 4 in the hexadecimal representation.

This is not equal to 0x3FD3333333333333 (the result of conversion from decimal 0.3, and also what would have been the result here if the rounding was specified the other way.)

Therefore, floating point 0.1 + 0.2 != 0.3.