A: 

because 0.1 is tricky
it's irrational recurring in binary (see comments)

EDIT
actually I'm winging it here, I don't know that for sure. Is there a Math.boffin out there who can disprove or provide a proof?
would be very interesting

meouw  2010-02-16 00:32:18
"Irrational" isn't true - 1 / 10 is a rational number regardless of what base you're storing things in. However, it *is* a recurring binary, so it's not possible to represent it exactly using a normal floating-point representation.
Anon.  2010-02-16 00:37:35
0.1 in decimal is 0.00011001100110011... recurring in binary.
Phil Ross  2010-02-16 00:40:31
rational being representable as a fraction - silly me. Could you shed some light on why it is recurring?
meouw  2010-02-16 00:41:30
in binary, each digit past the dot represents 1/2, 1/4, 1/8, 1/16, just as in decimal it is 1/10, 1/100, 1/1000. Using just these binary fractions, you can't ever add up to 1/10 exactly. The closest you can get is 1/16 + 1/32 + 1/256 + 1/512 + .... (0.0001100110011...)
nickf  2010-02-16 00:46:47
IEEE standards (754) dictate fractional numbers be stored this way.
littlefoot  2010-02-17 18:48:56
+2  A: 

In Firefox and Chrome, 0.1 + 0.2 is 0.30000000000000004, which is not equal to 0.3. This is presumably caused by it not being possible to represent 0.1 exactly as a floating point number.

Phil Ross  2010-02-16 00:32:27

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